常a,b,c,a1,a2,...,an>0
(a+b)/2≥√ab
a^2+b^2≥2ab
(a+b+c)/3≥(abc)^(1/3)
a^3+b^3+c^3≥3abc
(a1+a2+…+an)/n≥(a1a2…an)^(1/n)
2/(1/a+1/b)≤√ab≤(a+b)/2≤√[(a^2+b^2)/2]
n/(1/a1+1/a2+…+1/an)≤(a1a2…an)^(1/n)≤(a1+a2+…+an)≤√[(a1^2+a2^2+…an^2)/n]
|x1|-|x2|≤|x1+x2|≤|x1|+|x2|
|x1|-|x2|-…-|xn|≤|x1+x2+…xn|≤|x1|+|x2|+…+|xn|