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5.1 Number Theory

# 5.1 Number Theory - 5.1NumberTheory: . :N=cfw_1,2,3....

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5.1 Number Theory:  Prime and Composite Numbers Number theory is primarily concerned with the properties of numbers used for counting.  The counting numbers are also  called the natural numbers:  N = {1, 2, 3, …}.  The natural numbers that are multiplied together resulting in a particular  product are  factors  of that product.  If  a  and  b  are natural numbers,  a  is divisible by  b , or  b  is a factor or divisor of  a,   if  ÷   leaves a remainder of zero.  This is symbolized by  b a b  divides  a.    Rules of divisibility: 2 3 4 5 6 8 9 10 12 Last digit is 0, 2, 4, 6, or 8 Sum of the digits is divisible by 3 Last 2 digits are divisible by 4 Number ends in 0 or 5 Number is divisible by 2 and 3. Last 3 digits are divisible by 8. Sum of the digits is divisible by 9. Number ends in 0. Number is divisible by 4 and 3.

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