x^2-ax+a=0
x1+x2=a
x1*x2=a
按理说,两数之和=两数之积时,这两数相等,且值为2,即
x1=x2=2
a=4
(x1^2+x2^2)/(x1+x2)
=[(x1+x2)^2-2x1*x2]/a
=(a^2-2a)/a
=a-2
=2