sin(a+三分之派)+sina=-五分之四倍根号3,a在第四象限,求cosa

原式=2sin(α+π/6)cos(π/6)=-(4/5)*√3=sin(α+π/6)=-4/5 所以cos(α+π/6)=3/5cosa=cos(α+π/6-π/6)=cos(α+π/6）cos(π/6)+sin(α+π/6）sin(π/6)=(3√3-4)/10