解：X^2+2X-9=0(X+1)^2=10x+1=正负根号10x=正负根号10-1
配方（x+1）^2-10=0（x+1）^2=10x+1=正负根号10x=正负根号10-1
x^2+2x=9x^2+2x+1=9+1(x+1)^2=10x+1=正负根号10x1=根号10-1 x2=负根号10-1
X^2+2X-9=0(X+1)^2=10X=-1±根号10
