若 cos2a/sin(a-π/4)= -√2/2,求cosa+sina的值.
问题描述:
若 cos2a/sin(a-π/4)= -√2/2,求cosa+sina的值.
答
cos2a/sin(a-π/4)= -√2/2,sin(π/2-2a) /sin(a-π/4)= -√2/2,-sin(2a-π/2) /sin(a-π/4)= -√2/2,[-2 sin(a-π/4)cos(a-π/4)) /sin(a-π/4)= -√2/2,-2 cos(a-π/4))= -√2/2,-2[cos a cosπ/4+sin a ...