∵a∈(30°,90°) ∴a-30°∈(0°,60°) 又∵cos(a-30°)=15/17 ∴sin(a-30°)=8/17 ∴sina=sin(a-30°+30°)=sin(a-30°)cos30°+cos(a-30°)sin30° =8/17×√3/2+15/17×1/2=(8√3+15)/34