题目错误
当a＝60°,b＝30°时,左边＝0
右边＝cos120°－cos60°＝－1
应改成：2cos(a+b)*cos(a-b)=cos2b+cos2a
右边=cos2b+cos2a
=cos[(a+b)-(a-b)]+cos[(a+b)+(a-b)]
=cos(a+b)cos(a-b)+sin(a+b)sin(a-b)+cos(a+b)cos(a-b)-sin(a+b)sin(a-b)
=2cos(a+b)cos(a-b)