7x²-5=x+8
7x²-x-5-8=0
7x²-x-13=0
所以x1+x2=1/7,x1x2=-13/7
x1²+x2²=x1²+2x1x2+x2²-2x1x2=（x1+x2）²-2x1x2=（1/7）²-2（-13/7）=1/49+26/7
1/x1+1/x2=x2/x1x2+x1/x1x2=(x2+x1)/x1x2=(1/7)/(-13/7)=-1/13