Algebra Demystified 2nd Ed

469

Appendix

Factoring with Prime

Numbers

Factoring is a skill that is developed with practice. The only surefire way to fac-

tor numbers into their prime factors is by trial and error. There are some num-

ber facts that will make our job easier. Some of these facts should be familiar.

• If a number is even, the number is divisible by 2.


• If a number ends in 0 or 5, the number is divisible by 5.


• If a number ends in 0, the number is divisible by 10.


• If the sum of the digits of a number is divisible by 3, then the number is
  divisible by 3.


• If a number ends in 5 or 0 and the sum of its digits is divisible by 3, then
  the number is divisible by 15.


• If a number is even and the sum of its digits is divisible by 3, then the
  number is divisible by 6.


• If the sum of the digits of a number is divisible by 9, then the number is
  divisible by 9.


• If the sum of the digits of a number is divisible by 9 and the number is
  even, then the number is divisible by 18.