Question

Sin x-sin y=2/3 cos x-cos y=1/2 求cos(x-y)

Answer 1

Sin x-sin y=2/3

cos x-cos y=1/2

分别平方得

(Sin x-sin y)^2=(2/3)^2

(cos x-cos y)^2=(1/2)^2

展开相加得

-2cos(x-y)+2=4/9+1/4

-2cos(x-y)=1/2

cos(x-y)=-1/4