integration by substitution

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**(a)** Let $\displaystyle u=x^4$ then $\displaystyle du=4x^3dx$ and therefore $\displaystyle \frac{du}{4x^3}=dx$. We get:

$\displaystyle \int^b_{a} f(x^4)dx= \int^{1^4}_{(-1)^4} 1+u \frac{du}{4x^3} =\int^{1}_{1} 1+u \frac{du}{4x^3} $

Both limits of integration in the new integral are 1, so the value of the definite integral is 0.

**(b)**

$\displaystyle \frac{du}{dx}=4x^3$

It is only zero when x=0. Jacobian become zero at (0,0) so it vanishes at this point.

**(c)** This is the part I'm really stuck on: from part (a) I know what to substitute for "dx" when I make the substitution $\displaystyle u=x^4$. So I think this is what the question is asking for:

$\displaystyle \int^0_{-1} 1+u \frac{du}{4x^3} + \int^1_0 1+u \frac{du}{4x^3}$

But expressions involving x should not appear in the new integrals. How do I get rid of the "$\displaystyle 4x^3$"? Can anyone show me how to do this part?