sin[arc*sin(3/5)+arc*sin(7/18)]
先计算sin(arc*sin(3/5))=3/5,sin(arc*sin(7/18))=7/18
cos(arc*sin(3/5))=4/5,cos(arc*sin(7/18))=5√11/18

sin[arc*sin(3/5)+arc*sin(7/18)](两角和公式用上)
=3/5*5√11/18+4/5*7/18
=(15√11+28)/90