log以a为底b的对数+ log以b为底a的对数=log以a为底b的对数+1/（log以a为底b的对数）=-[-log以a为底b的对数-1/（log以a为底b的对数）]
然后运用均值不等式：若x>0则x+1/x>=2,当x=1时取等.