Question

若cosxcosy+sinxsiny=1/3,则cos(2x-2y)=

Answer 1

解：
cosxcosy+sinxsiny=cos(x-y)=1/3
又由cos2a=2cos²a - 1
故cos(2x-2y)=2*1/9 -1=-7/9
求解毕
望采纳

Answer 2

cosxcosy+sinxsiny=cos(x-y)=1/3,
令(x-y)=t
则cost=1/3
所以cos(2x-2y)=
cos2(x-y)
=cos2（x-y）的平方-1
=2cost的平方-1
=2*(1/3)-1=-7/9 答案是负九分之七