不懂再问我
sin2a >0 => 0<2a 0 sina >0cos(2a)=正负根号(1-(24/25)^2) = 正负7/25cos2a = 1-2(sina)^2当cos2a = 7/25时，2(sina) ^2 = 18/25 => sina =3/5cos2a = -7/25时，2(sina)^2 = 32/25 => sina = 4/5所以sina=3/5或者4/5
