sin(a -π/6 )=1/3,sinacosπ/6-cosasinπ/6=1/3√3sina/2-cosa/2=1/3√3sina-cosa=2/3√3sina=coaa+2/3两边平方3(sina)^2=(cosa)^2+4cosa/3+4/93-3(cosa)^2=(cosa)^2+4cosa/3+4/9令x=cosa4x^2+4x/3-23/9=036x^2+12x...