(1)
以下全是角度；
sin(-1380)=sin(-1380+1440)=sin60
cos1110=cos(1110-1080) =cos30=sin60
cos(-1020)=cos(-1020+1080) =cos60
sin750=sin30=cos60
原式=sin^2(60)+cos^2(60)=1
(2)
cos(2π/3+α)= - cos[π-(2π/3+α)]= - cos(π/3-α) = -√3/3
cos^2[7π/6+α]=sin^2[3π/2-(7π/6+α)]=cos^2(π/3-α)=1-(1/3)=2/3
原式= -√3/3+2/3=(2-√3)/3