Question

已知正实数a,b满足a+ b+ ab=8,则a+ 4b的最小值是?

Answer 1

设a+ 4b＝t
a=t-4b
a+ b+ ab=8
(t-4b)+b+b(t-4b)＝8
4b^2-(t-3)b+8-t=0
(t-3)^2-16(8-t)>=0
t^2-10t-119>=0
t>=17或t