sin^{-1} ( 3/ 5 ) = 0.6 435

You should express radians in ratio of pi

Sin^{-1 }( 3/5 ) = (0.6435/pi ) pi = 0. 2225 pi = pi/5

arcsin( 1/ √5 ) = 0.14758361765043*pi = 3pi/20 radians.