Question

已知a是第三象限角,sina=-12/13,求cos(派/6+a)

Answer 1

a是第三象限角,sina=-12/13

cosa=-根号(1-sin²a)=-根号(1-144/169)=-5/13

cos(派/6+a)=cosπ/6cosa-sinπ/6sina

=√3/2×(-5/13)-1/2×(-12/13)

=(12-5√3)/26