①√((a? +b? )/2)≥(a+b)/2≥√ab≥2/( 1/a+ 1/b).
②√(ab)≤(a+b)/2 .
③a? +b? ≥2ab .
④ab≤(a+b)? /4。
⑤||a|-|b| |≤|a+b|≤|a|+|b| .
Principle:
① The inequality f (x): F(x) has the same solution.
② If the inequality f (x)
③ If inequality f (x): 0, inequality f (x)
④ inequality F(x)G(x)>0 and inequality have the same solution; Inequality f (x) g (x)