ab=ab(1/a+1/b)=(a/b)+(b/a)+2≧4

1/a^2+1/b^2=(1/a+1/b)^2-2/(ab)=1-2/(ab)≧1/2

a^n・b＋a・b^n≧2√{(ab)^(n+1)}≧2^(n+2)