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Integrating exponential functions

karush

Well-known member
Jan 31, 2012
2,776
$\int x (5^{-x^2})dx= -\frac{1}{2} \int 5^{-x^{2}}(-2x)dx$

how is $\frac{1}{2}$ in front of the $\int$ derived
 
Last edited:

JJacquelin

New member
Jul 31, 2012
3
dx is missing !
5-x²=exp(-x²ln(5))
x²=t
x dx = dt/2
 

karush

Well-known member
Jan 31, 2012
2,776
sorry i hit the save instead of preview thot I cud fix it before reply
 

MarkFL

Administrator
Staff member
Feb 24, 2012
13,775
I've done that before, and it is annoying! (Headbang)
 

CaptainBlack

Well-known member
Jan 26, 2012
890
$\int x (5^{-x^2})dx= -\frac{1}{2} \int 5^{-x^{2}}(-2x)dx$

how is $\frac{1}{2}$ in front of the $\int$ derived
\[\int x (5^{-x^2})dx= \int x e^{-x^2\ln(5)}dx\]

Now: \[\frac{d}{dx} e^{-x^2\ln(5)}=-2x\ln(5) e^{-x^2\ln(5)}\] so:

\[\begin{aligned}\int x (5^{-x^2})dx &=-\frac{1}{2\ln(5)}\int \frac{d}{dx} e^{-x^2\ln(5)} dx\\
&=-\frac{e^{-x^2\ln(5)}}{2\ln(5)}+ C \\ &=-\;\frac{5^{-x^2}}{2\ln(5)}+C \end{aligned} \]

CB