第2个回答 2019-04-20
S
=1/(3a+2) + 1/(3b+2)
=1/(3a+2) + 1/[3(1-a)+2]
=1/(3a+2) + 1/(5-3a)
S'
=-3/(3a+2)^2 + 3/(5-3a)^2
S'=0
-3/(3a+2)^2 + 3/(5-3a)^2 =0
(5-3a)^2 -(3a+2)^2 =0
9a^2-30a+25 -(9a^2+12a+4)=0
-42a+21=0
a=1/2
S'|a=1/2+ >0
S'|a=1/2- <0
a=1/2 (min)
S= 1/(3a+2) + 1/(5-3a)
min S
=S(1/2)
=1/(3/2+2) + 1/(5-3/2)
=2/7 + 2/7
=4/7本回答被网友采纳