利用基本不等式:2√ab ≤a+b
(√a+√b+√c)^2
=a+b+c+2√ab+2√bc+2√ca
=1+2√ab+2√bc+2√ca
≤1+[(a+b)+(b+c)+(c+a)]
=1+2[a+b+c]
=3
∴√a+√b+√c≤√3