（x2+1）/（x1+1）+(x1+1)/(x2+1)
=（x2+x1x2）/（x1+x1x2）+(x1+x1x2)/(x2+x1x2)
=x2（x1+x2）/[x1（x1+x2）]+x1（x1+x2）/[x2（x1+x2）]
=x2/x1+x1/x2
=(x2^+x1^2)/（x1x2）
=[(x1+x2)^2-2x1x2]/(x1x2)
=9-2
=7