函数y=2cos(2x+a)是奇函数则-2cos(2x+a)=2cos(-2x+a)-cos2xcosa+sin2xsina=cos2xcosa+sin2xsinacos2xcosa=0上式恒成立,有cosa=0a=(k+1/2)π（B、D排除）当a=-π/2y=2cos(2x-π/2)-π/2
