1/2(cos2a-cos2b)=sin(a+b)sin(a-b)吗?
问题描述:
1/2(cos2a-cos2b)=sin(a+b)sin(a-b)吗?
答
不等于sin(a+b)sin(a-b),应该等于-sin(a+b)sin(a-b)
(1/2)(cos2A-cos2B)
=(1/2){cos[(A+B)+(A-B)]-cos[(A+B)-(A-B)]}
=(1/2)[cos(A+B)cos(A-B)-Sin(A+B)sin(A-B)]
-(1/2)[cos(A+B)cos(A-B)+sin(A+B)sin(A-B)]
=(1/2)sin(A+B)sin(A-B)+(1/2)sin(A+B)sin(A-B)
=-sin(A+B)sin(A-B)