初三数学第二题

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