yego.me
💡 Stop wasting time. Read Youtube instead of watch. Download Chrome Extension

Derivatives of sin(x) and cos(x) | Derivative rules | AP Calculus AB | Khan Academy


3m read
·Nov 11, 2024

What I'd like to do in this video is get an intuitive sense for what the derivative with respect to x of sine of x is and what the derivative with respect to x of cosine of x is. I've graphed y is equal to cosine of x in blue and y is equal to sine of x in red. We're not going to prove what the derivatives are, but we're going to know what they are and get an intuitive sense. In future videos, we'll actually do a proof.

So let's start with sine of x. The derivative can be viewed as the slope of the tangent line. So for example, at this point right over here, it looks like the slope of our tangent line should be zero. So our derivative function should be zero at that x value. Similarly, over here, it looks like the derivative is zero; the slope of the tangent line would be zero. So whatever our derivative function is at that x value, it should be equal to zero.

If we look right over here on sine of x, it looks like the slope of the tangent line would be pretty close to 1. If that is the case, then in our derivative function, when x is equal to 0, that derivative function should be equal to one. Similarly, over here, it looks like the slope of the tangent line is negative one, which tells us that the derivative function should be hitting the value of negative one at that x value.

So you're probably seeing something interesting emerge everywhere. While we’re trying to plot the slope of the tangent line, it seems to coincide with y is equal to cosine of x. And it is indeed the case that the derivative of sine of x is equal to cosine of x. You can see that it makes sense, not just at the points we tried, but even in the trends. If you look at sine of x here, the slope is one, but then it becomes less and less positive all the way until it becomes zero.

Cosine of x, the value of the function is one, and it becomes less and less positive all the way until it equals zero. You could keep doing that type of analysis to feel good about it. In another video, we're going to prove this more rigorously.

So now let's think about cosine of x. Cosine of x right over here, the slope of the tangent line looks like it is zero, and so its derivative function needs to be zero at that point. So hey, maybe it's sine of x. Let's keep trying this.

So over here, cosine of x looks like the slope of the tangent line is negative one, and so we would want the derivative to go through that point right over there. All right, this is starting to seem; it doesn't seem like the derivative of cosine of x could be sine of x. In fact, this is the opposite of what sine of x is doing. Sine of x is at one, not negative one at that point. But that's an interesting theory: maybe the derivative of cosine of x is negative sine of x.

So let's plot that. So this does seem to coincide. The derivative of cosine of x here looks like negative one, the slope of the tangent line, and negative sine of this x value is negative one. Over here, the derivative of cosine of x looks like it is zero, and negative sine of x is indeed zero.

So it actually turns out that it is the case that the derivative of cosine of x is negative sine of x. So these are really good to know. These are kind of fundamental trigonometric derivatives to know. We'll be able to derive other things for them, and hopefully, this video gives you a good intuitive sense of why this is true. In future videos, we will prove it rigorously.

More Articles

View All
2015 AP Physics 1 free response 1c
Let’s now tackle part C. They tell us block three of mass m sub 3, so that’s right over here, is added to the system, as shown below. There is no friction between block three and the table. All right, indicate whether the magnitude of the acceleration of …
AI and bad math
What we’re going to see in this video is that the current versions of artificial intelligence are not always perfect at math, and we’re going to test this out. I created a simple math tutor on Chat GPT here, and what we’re going to do is see if it can hel…
Private vs first class.
If you had the choice between flying private or flying first class, which would you choose? Private, 100% of the time. Flexibility, security, safety, quality of life, time. You can leave when you want to go, what airport you want to go to and from. It’s …
How To Save 99% Of Your Income
What’s up you guys? It’s Graham here. So, I thought this would be fun to get back to the basics and cover every technique that I have used along the way that’s allowed me to save nearly 100 percent of my income and essentially live for free. That includes…
Beautiful “Underwater Kaleidoscope” | National Geographic
I was inspired to be an ocean explorer from a very young age. We had a swimming pool in my backyard, and I would put on a little mask and fins and pretend I was Jacques Cousteau or I was swimming with sharks or dolphins or something. I had somewhat of an …
Tuna Gods Sacrifice | Wicked Tuna
You know, I don’t remember marking so many fish coming. That downline not bitin’. I have to catch fish because I have responsibilities on land. You know, my kids depend on me. I have tuition to deal with, so it really takes a tremendous toll mentally on t…