Question

若为正数a.b,ab=a+b+3.求ab的最小值,

Answer 1

ab-a-b=3

(a-1)(b-1)

=ab-a-b+1

=4

ab=(a-1)+(b-1)+5≥2√［（a-1)(b-1)］+5

因为（a-1)(b-1)=4

当且仅当a-1=b-1=2时

有最小值4+5=9