Question

已知sin(x/2)+cos(x/2)=1/2 ,求cos(2x)的值

Answer 1

解

sin(x/2)+cos(x/2)=1/2

两边平方

[sin(x/2)+cos(x/2)]²=1/4

即

1+sinx=1/4

∴sinx=-3/4

cos2x=1-2sin²x

=1-2×9/16

=1-9/8

=-1/8