# Geometric Proofs of Trigonometric Identities

Sparked by a conversation this past weekend about the usefulness of the half-angle identities, I constructed geometric proofs for $\sin(x/2)$ and $\cos(x/2)$. Since I’ve never seen these anywhere before, I thought I’d share.

And while I was at it, I thought I’d share all my other geometric proofs, so here they are, posted mostly without comment.

Some of these are so well-known as to be not worth mentioning. Many of them have been stolen from Proofs Without Words I or Proofs Without Words II. I came up with a few of them myself. Frustratingly, almost none of them are to be found in Precalculus textbooks, where they might be learned and appreciated.

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Though this one is my favorite:

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Partially because of the way it naturally generalizes into the proof of the derivative of sine. If you just let $\beta$ approach 0, $\cos(\beta)$ approaches 1 and that point in the interior of the circle ends up on the circle, where $\sin(\beta)$ merges with $\beta$ itself.

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And finally, one that shows that the sum of a sine and cosine function of the same argument is also a sinusoid. Since I lost the original picture and don’t feel like remaking it, you’ll have to complete the proof on your own!

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Update: After some feedback on twitter, I’ve decided to add a few more diagrams. Tim Brzezinski sent me a link to his website of geometric proofs of trig identities and he had some that I’ve never seen before.

Check it out!

https://www.geogebra.org/m/DxAcj8E2#material/QedMT7Pw

I’ve taken two of his diagrams and added them below.

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# National Math Festival 2017

There was mathematical mayhem in DC on Saturday!

Did you miss it? Let me try to capture the day with some photos:

That’s just ONE room, just one part of a very large and increasingly popular National Math Festival.

This was the second festival which is held every two years (alternating with the the US Science and Engineering Festival). The festival was a huge success and was very well attended. I was a little cautious about attendance predictions, given that the festival moved to the convention center from the DC Mall–a location which benefited from wandering foot-traffic.

This year, however, we benefited from the rain. It was dark and rainy all day long, but the National Math Festival provided a wonderful rainy-day escape from the dreary weather. See? Look at all the fun we’re having!

The photos you’re seeing here are all from the travelling exhibits brought to us by the Museum of Mathematics in NYC. I helped MoMATH coordinate volunteers this year, just as I did two years ago. And our volunteers were AWESOME!

We engaged thousands of people throughout the course of the day in meaningful mathematical play. There is a great need for this kind of popular-focus on mathematics, illuminating the beauty, joy, and fun of mathematics, rather than the impression people have of difficulty and drudgery.

All my photos are MoMATH-focused, since that’s where I spent my day. You can find even more of my photos here. And you can see more coverage in my twitter feed. For example, here’s a little clip of some juggling-math:

Did you miss this year’s festival? Mark your calendars for April 2019 and make it a priority!

# 2017 Pi Day Puzzle Hunt Recap

Imagine 150 teens sleuthing around the school solving puzzles, skipping lunch every day to gain advantages over other teams, students voluntarily solving extremely difficult puzzles.

Welcome to the Third Annual RMHS Pi Day Puzzle Hunt. This year 36 teams competed for \$200 in prize money, trophies and swag, and of course, GLORY. 🙂

There were eight challenging puzzles this year. A mural maze had students visiting other murals throughout the school in order to obtain the URL that gained them access to the next puzzle. The puzzles took students online, to classrooms, lockers, and making phone calls. Teams also received a UV light during the hunt in order to reveal secret messages (or cryptograms that still required decryption!). This year we did a better job of making the puzzles start out easy and slowly get more difficult, so as not to discourage teams right away. Here are links to descriptions of all of the 2017 puzzles:

Each year we have tried to improve the hunt in substantial ways, including the appearance of “Stars” throughout the hunt that earned students extra points by rewarding teams that could find hidden elements of puzzle or solve daily bonus puzzles. We also made the prize money and trophies better this year.

We had some bumps in the road, but overall, the 2017 hunt was a success. Months of work, and now our third puzzle hunt is in the books.

For more details, including photos, videos, and the puzzles, visit the Pi Day Puzzle Hunt Website.

See you next year, kids!

# Derivatives of Trigonometric Functions

First, let’s present the standard approach. This is from the calculus textbook I teach out of.

This was, as far as I was concerned, the only possible proof. The pedagogical flexibility lay entirely in how to frame the question, how to get students to discover the fact on their own (via graphical techniques), and how to add extra meaning to the result.

The most important question, so I thought for years, was really how one introduces and understands the fact that $\lim_{x \to 0} \frac{\sin x}{x}=1$. Some textbooks introduce it more or less out of the blue as “an important limit to know” and prove it via the Squeeze Theorem. Others prefer to wait until halfway through the above proof, realizing only then that this limit is important and solving it with a purpose in mind. There is also a difference of opinion as to how much rigor is required to establish the key inequality, that $\sin \theta < \theta < \tan \theta$. My textbook uses an area argument, but others prove the inequality with a nested sequence of segment inequalities.

My personal preference is for students to encounter $\lim_{x \to 0} \frac{\sin x}{x}=1$ “naturally” by attempting to graph $y=\frac{\sin x}{x}$ in precalculus, along with other interesting functions like $y=x \cdot \sin x, y=x \cdot \cos x, y = x + \sin x, y = e^{-x} \cdot \sin x$, and $y = \sin(1/x)$. These are more or less exercises in recognizing the so-called “envelope” of the product or sum of a periodic function and another function and have various scientific applications. The very informal geometric argument for why $\lim_{x \to 0} \frac{\sin x}{x}=1$ that one encounters in precalc prepares one for the more formal proof in calculus via the Squeeze Theorem.

All of this hard work to prove that $\lim_{x \to 0} \frac{\sin x}{x}=1$ almost seems to make it the real theorem and leaves $\frac{d}{dx} [\sin x] = \cos x$ as a corollary.

By contrast, consider this:

I’m tempted to make no further comment, since this beautiful and striking diagram so thoroughly and clearly explains why the derivative of sine is cosine. Tiny changes in the sine of an angle are proportional to the cosine of that angle since the red arc length above is effectively a tangent to the circle. I would go so far as to say that until you see a diagram like this, you don’t even really understand the theorem at all. Why don’t we teach the derivative of sine this way? Why is this figure not in all the textbooks? I think I know the answers to these questions. The answers involve a long story about the history of calculus, the banishment of infinitesimals during the quest for rigor, and the abandonment of geometry as a satisfactory basis for analysis. But these diagrams are just too beautiful to give up and it’s cruel of us to keep them hidden from our students.

Here’s another calculus proof:

Compare this to the standard treatment you find in textbooks:

Which one of these proofs excites you? Which one makes you really feel like you understand the theorem and why it’s true?

I have created an entire series and I post them here without further comment.

# The Product Rule

At some point in every calculus class, we must discover and prove the product rule for derivatives. How a calculus teacher chooses to do this probably says a lot about their pedagogy and educational priorities.

Some teachers might simply write the rule on the board, expect students to accept it, and immediately launch into examples. Should we try to let the students discover the formula on their own? Should we perhaps lead them into a trap by suggesting that the derivative of a product of two functions is the product of the derivatives and let them find counterexamples? Should we state the theorem, but let the students try to prove it on their own? Should we perhaps have an entire mini-lesson on what it even means to have a product of two functions?

Should we try to motivate the entire discussion with a particularly intuitive pair of functions whose product has some real-world significance? Should we interpret the product of two functions geometrically, as the area of the corresponding rectangle? If properly motivated and explained, do we actually gain anything by doing the rigorous proof via limits?

## The Status Quo

As a foil, here is the introduction to and proof of the product rule from the textbook that I teach out of.

I understand that textbooks have limited space and are no substitute for a full curriculum, but I think we can all agree that this is awful. There is no motivating example and no geometric intuition is called upon. The author merely proves the theorem, dryly and without understanding or purpose. The author even admits that the proof is unsatisfying and unedifying and apologizes in advance for its opaque maneuvers! Some proofs involve “clever steps that may appear unmotivated to a reader”.

In other words, reader, I am clever and you are not. This proof crucially involves cleverness, but since you’re not clever, you never would have thought of it yourself. I will perform some algebraic manipulations here in blue — they may appear unmotivated to you, but that’s your fault. In fact, I haven’t motivated them at all, but I don’t need to explain my clever methods to you. This is a calculus textbook after all, not a motivational textbook on explaining one’s cleverness. I have proved the rule, what else do you want me to do? If you want meaning and understanding, please consult your local religious figures for guidance.

Can we do better? Yes, I think we can. My friend James Key and I have used the phrase “tyranny of the blue text” to refer to totally opaque and unmotivated algebraic moves in textbook math proofs, since the offending expressions are often rendered in blue. Proving an important theorem to students via seemingly arbitrary, unmotivated algebraic tricks is an intellectual crime, and we should endeavor to banish the tyranny of the blue text from our classrooms and from our consciousness.

## Idea #1: A Word Problem

Suppose a particular factory produces toys 24 hours a day.

Let $W(t)$ be a continuous model of the number of workers at the factory at time $t$. The value of this function fluctuates throughout the day as workers leave and arrive according to their various particular schedules.

Let $E(t)$ be a continuous model of the number of toys produced per worker per hour at time $t$. This function measures the overall efficiency of the factory at a particular time of day. This could reasonably be expected to fluctuate due to external factors like the electricity supply, the weather (solar panels!), or the tiredness of the workers.

Then $(WE)(t) = W(t) \cdot E(t)$ is the total rate at which the factory produces toys, measured in toys per hour, at a particular time $t$.

$W'(t)$ is the rate of change of $W$ with respect to $t$, in other words the rate at which the workforce at the factory is rising or falling, as workers leave and arrive.

$E'(t)$ is the rate of change of $E$ with respect to $t$, in other words the rate at which the efficiency of the factory (on a per worker basis) is changing at a particular time $t$.

$(WE)'(t)$ is the rate at which the factory’s output is changing, at a given time $t$. In other words, if $(WE)'(t)$ is positive and big, the factory’s output is increasing a lot at that moment, but if $(WE)'(t)$ is positive and small, the factory’s output is increasing only a little at that time $t$.

Using our own common sense, what should $(WE)'(t)$ depend on? Surely $W'(t)$ is relevant, since even if efficiency holds steady, if workers are pouring into the factory at time $t$, the factory’s output will go up. But surely $E'(t)$ is also relevant, since even if the workforce holds steady, if the workers are becoming more efficient, then the factory’s overall output will go up. But the current size of the workforce, $W(t)$, is also relevant, since if, for example, efficiency is going up but the current workforce is very small, those gains in efficiency will not translate into large increases in output. And the current efficiency, $E(t)$, is also relevant, since if, for example, workers are pouring into the factory, but the current toy production per worker per hour is very small, then those extra workers will also not translate into large increases in output.

Just by having these conversations, we prime our students to have a deep appreciation of what the product rule is about, what differentiation is about, why we would ever want to multiply two functions, and why we would ever want to learn calculus.

This year, when giving this exact introduction to the product rule, I had a student guess the product rule right there on the spot, just from talking out the logic of the toy factory.

## Idea #2: A Geometric Interpretation

Some calculus textbooks motivate the product rule geometrically, by interpreting the product of two functions as the area of the rectangle whose side lengths are the values of the two functions at a given time.

This sloppy picture is taken from a presentation I gave at an NCTM conference a few years ago about calculus proofs. The area of the rectangle with side lengths $f$ and $g$ represents the value of the $fg$ function at a particular time. A moment later, both $f$ and $g$ change, and the derivative wants to measure the size of the change. Here again, we can read the product rule directly off the diagram. A “proof” like this was probably totally sufficient to a mathematician of the 18th century, but in a post-Cauchy/Weierstrass world, we need to verify these intuitions via the definition of the derivative as a limit.

But we can hold onto our geometric intuition and have our rigor as well!

The same diagram can be used to interpret that mysterious numerator in the definition of the derivative and avoid the tyranny of the blue text. The diagram motivates, but the rigor is preserved, since the limit just under the rectangle can be expanded and verified to be equivalent to the limit just above the diagram. But this time we are doing the proof with meaning and understanding.

Teaching the product rule this way might even be considered “standard”. The only drawback is that you do kind of have to be clever to think to do all this! Would a student come up with the idea to make a rectangle on their own? I’m not sure. I don’t claim to be the first or the only one to use a rectangle to discover, motivate, and even prove the product rule, but the following is something I have never seen anywhere before and that I just came up with a month ago. It is the excuse to write this blog post.

I’m getting a bit tired, so I pasted this picture in. The idea is simple. Combine a function with a known derivative and a generic second function $f$. And then just try to find the derivative of the product. No understanding or geometric intuition is required, but no teacher help or input is probably required either.

A reasonable calculus student who is confident, good at algebra, and experienced with limits and computing derivatives from the definition should be able to get to the end by just doing what comes natural.

I have not tried this before in class, so I can’t say how well it will work. But if it works, then the students have proved themselves to be every bit as clever as is required, and they’ve done it on their own. The teacher can then add extra layers of understanding to the general phenomenon of the product rule and lead the class through the general proof, possibly using geometry as a guide. But the students who figured out this particular example will feel that they could have done the general case on their own. And they will be right.

# Pi Day Puzzle Hunt

I’ll post more later, but here’s a sneak preview of this year’s Pi Day Puzzle Hunt. We’ve been working on this for months, and it finally happens next week. Here we go!

# Area models for multiplication throughout the K-12 curriculum

Let’s take a look at area models, shall we?

My thesis today is that area models should be ubiquitous across the entire curriculum because mathematics is a sense making discipline. As math educators, we ought to encourage our students to take every opportunity to visualize their mathematics in an effort to illuminate, explain, prove, and bring intuition.

So let’s take a walk through the K-12 math curriculum and highlight the use of area models as they might apply to arithmeticalgebra, and calculus.

# Arithmetic

Students experience area models for the first time in elementary school as they work to visualize multi-digit multiplication. This can also be used for division as well, just running the logic in reverse–that is, seeking an unknown “side length” rather than an unknown area. And Base Ten Blocks can be used to help students understand the building blocks of our number system.

Here’s how you might work out $27\times 54$:

$27\times 54 = (20+7)(50+4)=(20)(50)+(20)(4)+(7)(50)+(7)(4)$

$27\times 54=1000+80+350+28=1458$

The advantage of using a visual model like this is that you can easily see your calculation and explain why constituent calculations, taken together, faithfully produce the desired result. If you do a “man on the street” interview with most users or purveyors of the standard algorithm, you would almost certainly not get crystal clear explanations for why it produces results. For a further discussion of area models for multi-digit multiplication, see this article, or read Jo Boaler’s now famous book Mathematical Mindsets.

# Algebra

In middle school, as students first encounter algebra, they may use area models to support their algebraic reasoning around multiplying polynomials. And in an Algebra 2 course they may learn about polynomial division and support their thinking using an area model in the same way they used area models to do division in elementary school. Here Algebra Tiles can be used as physical manipulatives to support student learning.

Here’s how you might work out $(x+4)(2x+3)$:

$(x+4)(2x+3)=(x)(2x)+(x)(3)+(4)(2x)+(4)(3)$

$(x+4)(2x+3)=2x^2+3x+8x+12=2x^2+11x+12$

Notice also that if you let $x=10$, you obtain the following result from arithmetic:

$14\times 23 = 200+110+12=322$

The Common Core places special emphasis on making such connections. I agree with this effort, even though I can also commiserate with fellow math teachers who say things like, “My Precalculus students still use the box method for multiplying polynomials!” We definitely want to move our students toward fluency, but perhaps we should wait for them to realize that they don’t need their visual models. Eventually most students figure out on their own that it would be more efficient to do without the models.

# Calculus

Later in high school, as students first study calculus, area models can be used to bring understanding to the Product Rule–a result that is often memorized without any understanding. Even the usual “textbook proof” justifies but does not illuminate.

Here’s an informal proof of the Product Rule using an area model:

The “change in” the quantity $L\cdot W$ can be thought of as the change in the area of a rectangle with side lengths $L$ and $W$. That is, let $A=LW$. As we change $L$ and $W$ by amounts $\Delta L$ and $\Delta W$, we are wondering how the overall area changes (that is, what is $\Delta A$?).

If the side length $L$ increases by $\Delta L$, the new side length is $L+\Delta L$. Similarly, the width is now $W+\Delta W$. It follows that the new area is:

$A+\Delta A=(L+\Delta L)(W+\Delta W)=LW+L\Delta W+W\Delta L+\Delta L\Delta W$

Keeping in mind that $A=LW$, we can subtract this quantity from both sides to obtain:

$\Delta A=L\Delta W+W\Delta L+\Delta L\Delta W$

Dividing through by $\Delta x$ gives:

$\frac{\Delta A}{\Delta x}=L\cdot\frac{\Delta W}{\Delta x}+W\cdot\frac{\Delta L}{\Delta x}+\frac{\Delta L}{\Delta x} \frac{\Delta W}{\Delta x} \Delta x$

And taking limits as $\Delta x\to 0$ gives the desired result:

$\frac{dA}{dx}=L\cdot\frac{dW}{dx}+W\cdot\frac{dL}{dx}$

# Conclusion

If you’re like me, you once looked down on area models as being for those who can’t handle the “real” algebra. But if we take that view, there’s a lot of sense-making that we’re missing out on. Area models are an important tool in our tool belt for bringing clarity and connections to our math students.

Okay, so last question: Base Ten Blocks exist, and Algebra Tiles exist. What do you think? Shall we manufacture and sell Calculus DX Tiles © ? 🙂