sin(54π/7)=sin(54π/7-8π)=sin(-2π/7)
sin(-63π/8)=sin(-63π/8+8π)=sin(π/8)
sin231=sin(180-231)=sin(-51)=sin(-51π/180)
cos260=sin(90-260)=sin(-170)=sin(-10)=sin(-π/18) 关于-180对称
-π/2~π/2单调递增为间,
所以
sin(-2π/7)