Question

sin12°cos18°-sin78°cos108°=?

高中数学 要过程

Answer 1

sin12°cos18°-sin78°cos108°=sin12°cos18°-sin(90°-12°)cos(90°+18°)=sin12°cos18°+cos12°sin18°=sin(12°+18°)=sin30°=0.5

Answer 2

原式=sin12cos18-cos12(-cos72)
=sin12cos18+cos12sin18
=sun(12+18)
=sin30
=1/2