x^2-12x+m=(x+n)^2=x^2+2nx+n^2所以2n=-12m=n^2n=-6m=36所以m-n=36+6=42
x^2-12x+m=(x+n)^2=X²+2XN+N²则：待定系数法得 -12=2N;N=-6;N²=36=M;M=6或M=-6M-N=0或M-N=-12
