化简 sin^2 a+cos a*cos（60+a）-sin^（30-a）=

sin^2 a+cos a*cos（60+a）-sin^2（30-a)=sin^2 a+cosa(cosa*cos60-sina*sin60)-(sin30*cosa-sina*cos30)^2=sin^2 a+1/2cos^2 a-厂3/2sina*cosa-1/4cos^2 a+厂3/2sina*cosa-3/4sin^2 a=1/4sin^2 a+1/4cos^2 a=1/4...