二元一次方程都是要化为一元一次方程的。l + r = 7 r = 10/ll + 10/l = 7l^2 -7l +10 = 0因式化解 l=2或l=5即，l=2,r=5 或l=5,r=2
l+r=7l=7-r,代入lr=10(7-r)r=10-r^2+7r-10=0(-r+2)(r+5)=0r=2或r=-5r=2，l=5r=-5,l=12
