# Gaussian Integral Substitution

 P: 8 If I had an integral $$\int_{-1}^{1}e^{x}dx$$ Then performing the substitution $x=\frac{1}{t}$ would give me $$\int_{-1}^{1}-e^\frac{1}{t}t^{-2}dt$$ Which can't be right because the number in the integral is always negative. Is this substitution not correct? Sorry if I am being very thick but I can't figure out why I can't evaluate this simple integral with this change of variables.
HW Helper
Thanks
P: 26,160
hi pcvrx560!
 Quote by pcvrx560 … Is this substitution not correct?
the substitution is fine, but the limits are wrong …

as x goes up from -1 to 1,

t (= 1/x) goes down from -1 to -∞, and then from +∞ down to 1 …

you'ld need to write ##\int_{-1}^{-\infty} + \int_{\infty}^{1}##
(or ##-\int^{-1}_{-\infty} - \int^{\infty}_{1}##)
 Emeritus Sci Advisor HW Helper Thanks PF Gold P: 6,301 Isn't the integral of e^x w.r.t. x simply e^x + C?
 P: 8 Gaussian Integral Substitution Thanks, tiny-tim! That cleared it up for me. I didn't think about the range I was integrating over, I was just mindlessly plugging numbers into 1/t.

 Related Discussions Calculus & Beyond Homework 4 Calculus & Beyond Homework 4 Calculus & Beyond Homework 3 Calculus & Beyond Homework 4 Calculus & Beyond Homework 4