960÷X-960÷（X+20）=4
去分母得 960(x+20)-960x=4x(x+20)
960x+19200-960x=4x^2+80x
4x^2+80x-19200=0
x^2+20x-4800=0
(x+60)(x-80)=0
x1=-60,x2=80