设a,b,c均为正实数,求证1/2a+1/2b+1/2c≥1/(a+b)+1/(b+c)+1/(a+c)
问题描述:
设a,b,c均为正实数,求证1/2a+1/2b+1/2c≥1/(a+b)+1/(b+c)+1/(a+c)
答
(a-b)^2≥0
(a+b)^2≥4ab
1/4a+1/4b =(a+b)/4ab ≥(a+b)/(a+b)^2
1/4a+1/4b≥1/(a+b) (1)
同理 1/4a+1/4c≥1/(a+c) (2)
1/4b+1/4c≥1/(b+c) (3)
(1)+(2)+(3)得
1/2a+1/2b+1/2c≥1/(b+c)+1/(c+a)+1/(a+b)