sin(60-x)=1/4,cos(60+2x)=

题目

sin(60-x)=1/4,cos(60+2x)=

答案

sin(60-x)=1/4
则cos[90°-(60°-x)]=sin(60-x)=1/4
cos(30°+x)=1/4
cos(60°+2x)=cos2(30°+x)
=2cos²(30°+x)-1
=2*(1/4)²-1
=-7/8
即-8分之7