Principles of Countig

if a think can be done m1 ways,and after it has been done a second thing can be done in m2 ways,then the two things can be done in m1,m2 ways in the indicated order.In general, if teher are m1,m2,m3,m4,……,mn different ways to do n things respectively,then these n things can be done in m1,m2,m3,.  mn different ways.

Counting by summation if A>B=¢(DİSJOİNT SETS) n(A<B)=n(A)+n(B)

Example1: There are 80 mathematics and 45 biology books in bookshop. in how many different ways can we buy one mathematisc books or  biology book.

n(M)=80 and n(B)=45

n(m<b)= 80+45=125

Counting by product

example2 : There are 31 calasroom each containing 19 desk in a school. how many desks are there in this school?

19*31=229

example 3: There are 4 ways travelling from the city ankara to the city sivas, and there are 5 ways of travelling from city sivas to the city erzurum .inhow many different ways can we travel from the city ankara to the city erzurum stopping over at sivas.

m1=4,                  m2=5                       

     m1.m2=4*5=20