由题意可知：2x1²=3x1+1 x1+x2=3/2 x1x2=-1/2 2X1²+3X2-4X1X2-5=3x1+1+3x2-4x1x2-5=3*(x1+x2)+1-4x1x2-5=3 * 3/2 - 4 * (-1/2) -4=5/2
