解sin420*cos750+sin(-690)*cos(-660)=sin(420-360)*cos(750-2*360)+sin(-690+2*360)*cos(-660+2*360)=sin60*cos30+sin30*cos60由特殊角的三角函数值可很容易得出：原式=3/4+1/4=1此题主要是熟练运用公式sin(X±n*36...