Combining integrals with swapped limits

[jinksys]

! SOLVED !

This isn't a homework problem, just an equation in my chapter. I don't see how the two integrals pointed to by the blue arrows become the integral pointed to by the red arrow. I know that if you swap the limits of integration, you change the sign of the integral. However, how do you change the -pi to pi in the second integral?

 

[hunt_mat]

Use the substitution u=-x

 

[jinksys]

Of course, thanks.