好難既math啊...高手唔該幫下手啊!!!

In fact there is no need to use derivative =)
For calculate of minimum length, imagine a triangle like below, because minimum distance for two point A & B must be a straight line.
╴　Ａ
↑　│╲
│　│　╲
３　│　　╲
↓　│　　　╲
─　├────╲　Ｐ
↑　│　　　　　╲
２　│　　　　　　╲
↓　│　　　　　　　╲
─　├────────╲　Ｑ
↑　│　　　　　　　　　╲
８　│　　　　　　　　　　╲
↓　│　　　　　　　　　　　╲
─　└────────────╲　Ｂ
　　│←－－－－１０－－－－→│
　　│←－－Ｙ－－－→│
　　│←Ｘ－→│
X = 3/13 * 10 = 2.608
Y = 5/13 * 10 = 3.846
AP+PQ+QB=AB=sqrt(13^2+10^2)
=sqrt(269)
=16.401
有幾難呢！
I hope this can help with your understanding. =)

2007-11-17 10:33:12 補充：
The above calculation is based on Y is taken on the point QIf Y is not taken on the point Q, then the height of the triangle have to redistributed as 3 / 4 / 6X = 3/13 * 10 = 2.608Y = 7/13 * 10 = 5.385　　　　　　＾＾＾＾＾＾＾＾AP PQ QB=AB=sqrt(13^2 10^2)=sqrt(269)=16.401=)