高一三角函数习题若cos2a/sin(a-π/4)=-√2/2,则cosa+sina的值为 ( )A.-√7/2 B.-1/2 C.1/2 D.√7/2
问题描述:
高一三角函数习题
若cos2a/sin(a-π/4)=-√2/2,则cosa+sina的值为 ( )
A.-√7/2 B.-1/2 C.1/2 D.√7/2
答
由cos2α/sin(α-π/4)=-(√2)/2,且cos2α=cos²α-sin²αsin(α-π/4)=(√2)/2*(sinα-cosα)则:cos2α/sin(α-π/4)=cos²α-sin²α/((√2)/2*(sinα-cosα))=-(√2)/2等式左边同时约去sinα-co...