e^-2.5xdx .... i got -5/2e^2,5x+c
Xcosx(3x^2)dx.... i got 1/12sin(3x^2) + c
(5x/x^2+1)dx
X(x^2-8)^1/2 dx
(1-e^-x/e^-x)dx
(3x-2)^5/2 dx
I'm guessing you mean: $\displaystyle \int x\cos \left(3x^{2}\right) dx$
Check over your work again. The constant in front of your antiderivative is off. Show your work and we'll see if we can sort it out for you.
$\displaystyle u = x^{2} + 1$
$\displaystyle u = x^{2} - 8$
Split the fraction: $\displaystyle \frac{1-e^{-x}}{e^{-x}} = \frac{1}{e^{-x}} - \frac{e^{-x}}{e^{-x}}$
And simplify.
$\displaystyle u = 3x - 2$
It'd be more helpful if you posted your work so we can pinpoint your mistakes.