a∈(π/3,5π/6),则π/6+a∈(π/2,π)sin(π/6+a)=4/5,则cos(π/6+a)=-3/5cosa=cos[(π/6+a)-π/6] =cos(π/6+a)cos(π/6)+sin(π/6+a)sin(π/6) =(-3/5)(√3/2)+(4/5)(1/2) =(4-3√3)/10祝你开心...