=√3/2.方法一:(cos^2)15°-(sin^2)15=(cos15+sin15)(cos15-sin15)=[sin(90-15)+sin15][sin(90-15)-sin15]=(sin75+sin15)(sin75-sin15)=2*sin(75+15)/2*cos(75-15)/2*2*cos(75+15)*sin(75-15)/2=4*sin45*cos30*cos45...