2015 AP Calculus AB 6c

- [Voiceover] Part C, evaluate the second derivative of y with respect to x squared at the point on the curve where x equals negative one and y is equal to one. Alright, so let's just go to the beginning where they tell us that dy dx is equal to y over three y squared minus x. So let me write that down. So dy dy dx is equal to y over three y squared minus x. Three y squared minus x. And now what I wanna do is essentially, take the derivative of both sides again. Now, there's a bunch of ways that we could try to tackle it, but the way it's written right now, if I take the derivative of the right-hand side, I'm gonna have to apply the quotient rule, which is really just comes from the product rule, but it gets pretty hairy. So, let's see if I can simplify my task a little bit. I'm going to have to do implicit differentiation one way or the other, so what if I multiply both sides of this times three y squared minus x? Then I get three y squared minus x times dy dx is equal to, is equal to y. And so now, taking the derivative of both sides of this is going to be a little bit more straightforward. In fact, if I want, I could, well actually, let me just do it like this. And so, let's, let's apply our derivative operator to both sides. Let's give ourselves some space. So I'm gonna apply my derivative operator to both sides. We take the derivative of the left-hand side with respect to x, and the derivative of the right-hand side with respect to x. And so, here I'll apply the product rule. First, I'll take the derivative of this expression and then multiply that times dy dx, and then I'll take the derivative of this expression and multiply it times this. So, the derivative of three y squared minus x with respect to x, well, that is going to be, so if I take the derivative of three y squared with respect to y, it's going to be six y, and then I have to take the derivative of y with respect to x, so times dy dx. As we say, and this is completely unfamiliar to you and I encourage you to watch the several videos on Khan Academy on implicit differentiation, which is really just an extension of the chain rule. I took the derivative of three y squared with respect to y, and took the derivative of y with respect to x. Now, if I take the derivative of negative x with respect to x, that's going to be negative one. Fair enough. And so, I took the derivative of this, I'm gonna multiply it times that. So times dy dx and then to that, I'm going to add the derivative of this with respect to x. Well, that's just going to be the second derivative of y with respect to x times this. And all I do is apply the product rule. Three y squared minus x. Once again, I'll say it a third time, derivative of this times this plus the derivative of this times that. Alright. That's going to be equal to, on the right-hand side, derivative of y with respect to x is just dy dx. Now, if I want to, I could solve for the second derivative of y with respect to x, but what could be even better than that is if I substitute everything else with numbers because then it's just going to be a nice, easy numerical equation to solve. So, we know that, what we wanna figure out, our second derivative, when y is equal to one. So y is equal to one. So that's going to be equal to one and this is going to be equal to one. X is equal to negative one. So, let me underline that. X is equal to negative one. So x is negative one. Are there any other xs here? And what's dy dx? Well, dy dx, dy dx is going to be equal to one over three times one, which is three, minus negative one. So, this is equal to one fourth. In fact, that's I think where we evaluated in the beginning. Yep, at the point negative one comma one. At the point negative one comma one. So that's dy dx is one fourth. So this is one fourth, this is one fourth, and this is one fourth. And now, we can solve for the second can solve for the second derivative. Just gonna make sure I don't make any careless mistakes. So if six times one times one fourth is going to be one point five, or six, well, six times one times one fourth is six fourths, minus one is going to be, so six fourths minus four fourths is two fourths, so this is going to be one half times one fourth, times one fourth, that's this over here, plus the second derivative of y with respect to x, and here I have one minus negative one, so it's one plus one, so times two. So maybe I'll write it this way. Plus two times the second derivative of y with respect to x is equal to one fourth. And so let's see, I have one eighth plus two times the second derivative is equal to one fourth. And let's see, I could subtract one eighth from both sides. So let's subtract one eighth. Subtract one eighth. These cancel. I get two times the second derivative of y with respect to x is equal to, one fourth is the same thing as two eighths, so two eighths minus one eighth is one eighth. And then I can divide both sides by two, and we get a little bit of a drum roll here, we divide both sides by two, or you could say multiple both sides times one half, and you are going to get, you are going to get the second derivative of y with respect to x when x is negative one and y is equal to one, is one over 16. And we're done.