2（x+3）²=x²-9
2（x²+6x+9)=x²-9
2x²+12x+18-x²+9=0
x²+12x+27=0
(x+3)(x+9)=0
x1=-3,x2=-9用因式分解法2（x+3）²=x²-92（x+3)²=(x+3)(x-3)2(x+3)²-(x+3)(x-3)=0(x+3)[2(x+3)-(x-3)]=0(x+3)(x+9)=0x1=-3,x2=-9