椐正弦知碧定理吵尺:sin120°/(35T)=sina/搭碰举(25T)=sin(60-a)/15sin120°/(35T)=sina/(25T)sina=5sin120°/7=5√3/14=0.6186cosa=√(1-0.6186²)=0.7857sin120°/(35T)=sin(60-a)/15T=3sin120°/(7sin(60-a))=1h