已知cos(α-π/3)=15/17 ,α为钝角,求cosα
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已知cos(α-π/3)=15/17 ,α为钝角,求cosα
已知cos(α-π/3)=15/17 ,α为钝角,求cosα
已知cos(α-π/3)=15/17 ,α为钝角,求cosα
因为cos(a-π/3)=15/17>0,a∈(π/2,π)
则a-π/3=arccos15/17
a=arccos15/17+π/3
则
cosa=cos(arccos15/17+π/3)=cos(arccos15/17)cosπ/3-sin(arccos15/17)sinπ/3
=15/17*√3/2+8/17*1/2=(8+15√3)/34