∵sin^2(α)+cos^2(α)=1 ∴sin^2 123°=1-cos^2 123°∴原式=√(1-(1-cos^2 123°))=√cos^2 123°=√cos(90°+33°)= - sin 33°(∵ cos（π/2+α）= -sinα)PS: 好多年没有看过这个了.如有错漏,还请大家赐教...