已知a从属于(3π/4,π)若sin(a-(π/4))=3√10/10（1）求cos(a+(π/4))的值(2)求sin2a的值

Question

已知a从属于(3π/4,π)若sin(a-(π/4))=3√10/10
（1）求cos(a+(π/4))的值 (2)求sin2a的值

Answer 1

1、
cos(a+π/4)
=cos[π/2 +(a-π/4)]
= -sin(a-π/4)
= -3√10 /10
2、
sin2a
=cos(π/2-2a)
=1- 2sin²(π/4-a)
=1 - 18/10
= -4/5