x1,x2是方程2x²+5x-1=0的两个根
∴x1+x2=-5/2
x1x2=-1/2
x2/x1+x1/x2=(x2²+x1²)/x1x2=[(x1+x2)²-2x1x2]/(x1x2)=(25/4+1)/(-1/2)=-29/2[(x1+x2)²-2x1x2]/(x1x2）？？？？？这是什么啊x2²+x1²=x1²+2x1x2+x2²-2x1x2=(x1+x2)²-2x1x2为什么还要减2X1X2前面加一个2x1x2 目的是为了配方多加了2x1x2，就要减去2x1x2