M=3X*2-2XY-4Y*24M=12X*2-8XY-16Y*2N=4X*2+5XY-Y*24M-N=8X*2-13XY-15Y*2所以8x*2-13xy-15y*2=4M-N
8x*2-13xy-15y*2=4M-N
4倍根号m减根号n