分母=1-sin^2(15°)/cos^2(15°)=[cos^2(15°)-sin^2(15°)]/cos^2(15°)=cos2*15°/cos^2(15°)=cos30°/cos^2(15°)原式=tan15°/[cos30°/cos^2(15°)]=sin15°/cos15°/[cos30°/cos^2(15°)]=（sin15°*cos15°...