P(-3,4)∴x=-3,y=4.r=√x²+y²=5
sinα=y/r=4/5
cosα=-3/5
sin2α=2sinα·cosα=-12/25
(1) sin(π-2a)=sin2α=-12/25
(2) cos（a+π/3）=cosα·cosπ/3-sinα·sinπ/3
=(-3/5)×1/2-4/5×√3/2
= -(3+4√3)/10