# Question about functions

1. Sep 29, 2013

### Woopydalan

Hello,

I am curious, suppose I have a function of x, f(x). Suppose I also have another function P(x). Does this mean I am allowed to have f(P) and I can do standard methods of integration and such on it?

2. Sep 29, 2013

### Robert1986

If P is the variable that you are integrating. For example, $\int \sin(x^2) d(x^2)$ can be computed the way it seems you want to compute it. That is, $\int \sin(x^2) d(x^2) = - \cos(x^2)$. However, $\int \sin(x^2) dx \neq - \cos(x^2)$. The question you are asking has a lot to do with the integration method known as "u-substitution." Do you know what that is?

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