sin(a+30度)=1/3,则cos(2a-120度)的值是___________.

题目

sin(a+30度)=1/3,则cos(2a-120度)的值是___________.

答案

cos(2a-120)=-cos(2a-120+180)=-cos(2a+60)=-cos2(a+30)=-[1-2[sin(a+30)]^2]=-7/9