1、x+2÷x=3
x²+2=3x
x²-3x+2=0
(x-1)(x-2)=0
x=1或x=2
2、x+6÷x=5
x²+6=5x
x²-x+5=0
(x-1)(x-5)=0
x=1或x=5
3、x+12÷x=7
x²+12=7x
x²-7x+12=0
(x-3)(x-4)=0
x=3或x=4
而：x+(n²+n)÷(x-3)=2n+4
x(x-3)+(n²+n)=(2n+4)(x-3)
x²-3x+n²+n=2nx+4x-6n-12
x²-3x-2nx-4x+n²+n+6n+12=0
x²-(2n+7)x+n²+7n+12=0
x²-[(n+3)+(n+4)]x+(n+3)(n+4)=0
[x-(n+3)][x-(n+4)]=0
x=n+3或x=n+4