tích phân 12

D

dien0709

$I=\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}}\dfrac{cosxdx}{1+e^x}$

sai hok chịu tn

$x=-t\to I=\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}}\dfrac{e^tcostdt}{1+e^t}=\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}}\dfrac{e^xcosxdx}{1+e^x}$

$\to e^x=1\to x=0\to I=\int_{-\frac{\pi}{2}}^{\frac{\pi}{2}}\dfrac{1}{2}dx= \dfrac {\pi}{2}$
 
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