∫(0->π/2) xcos2xdx=(1/2)∫(0->π/2) xdsin2x=(1/2)[ x.sin2x]|(0->π/2) -(1/2)∫(0->π/2) sin2x dx=0-(1/2)∫(0->π/2) sin2x dx=(1/4)[ cos2x ]|(0->π/2)=-1/2
