已知向量A=(2cosa ,2sina )a∈(π/2,π),b=(0,-1),则向量a与b的夹角是 A.3π/2-a B.π/2+a C.a-π/2 D.a
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![已知向量A=(2cosa ,2sina )a∈(π/2,π),b=(0,-1),则向量a与b的夹角是 A.3π/2-a B.π/2+a C.a-π/2 D.a](/uploads/image/z/7151006-38-6.jpg?t=%E5%B7%B2%E7%9F%A5%E5%90%91%E9%87%8FA%3D%282cosa+%2C2sina+%29a%E2%88%88%EF%BC%88%CF%80%2F2%2C%CF%80%EF%BC%89%2Cb%3D%280%2C-1%29%2C%E5%88%99%E5%90%91%E9%87%8Fa%E4%B8%8Eb%E7%9A%84%E5%A4%B9%E8%A7%92%E6%98%AF+A.3%CF%80%2F2-a+B.%CF%80%2F2%2Ba+C.a-%CF%80%2F2+D.a)
已知向量A=(2cosa ,2sina )a∈(π/2,π),b=(0,-1),则向量a与b的夹角是 A.3π/2-a B.π/2+a C.a-π/2 D.a
已知向量A=(2cosa ,2sina )a∈(π/2,π),b=(0,-1),则向量a与b的夹角是
A.3π/2-a B.π/2+a C.a-π/2 D.a
已知向量A=(2cosa ,2sina )a∈(π/2,π),b=(0,-1),则向量a与b的夹角是 A.3π/2-a B.π/2+a C.a-π/2 D.a
cos=a*b/|a||b|=-2sina/2*1=-sina
∵a∈(π/2,π),∴-sina=cos(π/2+a)=cos
∴=π/2+a
∵cos 为偶函数,∴=-π/2-a=2π+(-π/2-a)=3π/2-a
∴选A