sinc=4/5 =>cosc=3/5或-3/5 当=3/5 c^2=a^2+b^2-2ab*cosc =9+25-2*3*5*3/5 =16 =>c=4 当=-3/5 =>c^2=9+25+2*3*5*3/5=52 c=根号52
