2009-11-14

counting with Venn Diagrams

How many arrangements of the letters in CHEMIST

have H before E, or E before T, or T before M?

(Here 'before' means anywhere before, not just immediately before.)

請助教指導


1 則留言:

AIdrifter 提到...

不知道對不對 參考看看

用排容

HE+ET+TM
-HET-ETM-TMHE
+HETM

(7!/2)*3
-(7!/3!)*2-(7!*(6/4!))
+(7!/4!)

TMHE(這個順序有6種)
TMHE MHTE
THME MTHE
THEM MTEH

HETM(這個只有一種)