# Derivative of a composite function

1. Dec 8, 2013

### Tensel

y =f(x), and y'=df(x)/dx, F(y)=y^3+(y')^2
how to deal with the (y')^2 when i calculate dF(y)/dy?
thanks.

2. Dec 8, 2013

### scurty

Do you remember the chain rule? Are you having trouble differentiating that term?

3. Dec 8, 2013

### HallsofIvy

Staff Emeritus
If u(x) is a fuction of x, then, by the chain rule, the derivative of $u^2$ is 2u times the derivative of u.

If y is a function of x, then the derivative of $(y')^2$, with respect to x, is $2y'$ times the derivative of y' which is, of course, y''. That is, the derivative if $(y')^2$ is $2y' y''$.

4. Dec 8, 2013

### Tensel

HallsofIvy, i want to calculate dF(y)/dy, not dF(y)/dx, but you remand me sth. thank you.

5. Dec 9, 2013

### HallsofIvy

Staff Emeritus
Well, that doesn't require any mention of x at all then! The derivative of $d(y)^2/dy= 2y$.