2400/x-2400/(x+40)=2
1200/x-1200/(x+40)=1
乘以x(x+40)得
1200(x+40)-1200x=x(x+40)
1200x+48000-1200x=x²+40x
x²+40x-48000=0
a=1  b=40   c=-48000
代入求根公式得
x=[-40±√(1600+192000)]/2=[-40±440)]/2
x1=-240   x2=200