sin(40°-α)=1/3则 sin(10°+2α)的值等于 A.7/9 B.-7/9 C.-2/9 D.2/9

问题描述:

sin(40°-α)=1/3则 sin(10°+2α)的值等于 A.7/9 B.-7/9 C.-2/9 D.2/9

cos(80-2a)
=1-2sin²(40-a)
=7/9
所以sin(10+2a)
=cos[90-(10+2a)]
=cos(80-2a)
所以选A