由f(x)的定義域?yàn)?0,+∞),x1>0,x2>0,
故x1+x2>x1,x1+x2>x2,由f(x)/x單調(diào)遞減,
故f（x1+x2)/(x1+x2) ≤ f(x1)/x1 =>x1*f(x1+x2)/(x1+x2) ≤ f(x1)
f(x1+x2)/(x1+x2) ≤ f(x2)/x2=>x2*f(x1+x2)/(x1+x2) ≤f(x2)
兩式相加得, (x1+x2)*f(x1+x2)/(x1+x2) ≤ f(x1)+f(x2),
故f(x1+ x2)≤f(x1)+f(x2)兩式相加得， (x1+x2)*f(x1+x2)/(x1+x2) ≤ f(x1)+f(x2)。怎么加出來的 你用筆下 拍下 發(fā)郵件給我吧kumoshuai@163.com由f(x)的定義域?yàn)?0,+∞)，x1>0，x2>0,故x1+x2>x1，x1+x2>x2，由f(x)/x單調(diào)遞減，故f（x1+x2)/(x1+x2) ≤ f(x1)/x1 =>x1*f(x1+x2)/(x1+x2) ≤ f(x1)①f(x1+x2)/(x1+x2) ≤ f(x2)/x2=>x2*f(x1+x2)/(x1+x2) ≤f(x2)②①+② 兩式相加得， (x1+x2)*f(x1+x2)/(x1+x2) ≤ f(x1)+f(x2)，(這一步是把①和②中相同的因式 f(x1+x2)/(x1+x2))提取出來，然后把①剩下的x1和②剩下的x2相加)故f(x1+ x2)≤f(x1)+f(x2) （這一步是把式子等號(hào)左邊的(x1+x2)*f(x1+x2)/(x1+x2)中分子，分母同除于(x1+x2)）