ab=1
a,b互为倒数.
1/a^2+1/b^2+1
=a^2+1/a^2+1
由均值不等式,得
a^2+1/a^2≥2 1/a^2+1/b^2+1≥2+1=3
当a=b=1或a=b=-1时,1/a^2+1/b^2+1有最小值,最小值为3