cos（-17π/4）=cos（17π/4）=cos（4π+1π/4）=cos（π/4）=根号2/2sin(-26π/3)=-sin(26π/3)=-sin(8π+2π/3)=-sin(2π/3) = -根号3/2
我回答（2）吧 原式=1+sina*sina-2cos^2a=sin^2a-cos2a
