a+b+c (a+b+c)^2= a^2 + b^2 + c^2 +2ab + 2bc + 2ac =b>=c所以2ab>=2b^2 2bc >=2c^2 2ac >= 2c^2因此有a^2 + b^2 + c^2 + 2ab + 2bc + 2ac >= a^2+3b^2+5c^2所以a^2+3b^2+5c^2=