cos(2x)= 2cos^2(x)-1
cos^2(x)= [1+cos(2x)]/2
同理cos^2(x/2)= [1+cos(x)]/2
∫cos^2(x/2)dx= ∫[1+cos(x)]/2 dx =∫1/2 dx + ∫cos(x)/2 dx
= x/2 + sin(x)/2 + C