sin(a-b)cosa-cos(a-b)sina=3/5求cos2b

问题描述:

sin(a-b)cosa-cos(a-b)sina=3/5求cos2b

sin(a-b)cosa-cos(a-b)sina=3/5=sin(a-b-a)=-sinb
cos2b=1-2sin^2b=1-2*(3/5)^2=7/25

sin(a-b)cosa-cos(a-b)sina=sin(a-b-a)=-sinb=3/5 cos2b=1-2sinb∧2=1-18/25=7/25