a^2+ma+1/9=(a+n)^2=a^2+2na+n^2
n^2=1/9,n=±1/3
m=2n=±2/3,且与n符号相同
1/3x^2-3=1/3(x^2-9)=1/2(x+3)(x-3)
a^2-10a+(25)=(a-5)^2
9x^2+kx+16=(3x+4)^2=9x^2+24x+16,k=24
x^3-xy^2=x(x^2-y^2)=x(x+y)(x-y)
x^3-25x=x(x^2-25)=x(x+5)(x-5)
x^3-6x^2+9x=x(x^2-6x+9)=x(x-3)^2