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The expression 6 × 5 × 4 × 3 × 2 × 1 = can be written as 6!, which is read as “six factorial.”

In general, n! is the product of all the counting numbers beginning with *n* and counting backwards to 1. We define 0! to be 1.

* Example*

Find the value of each expression:

a) 3!

b) 0!

c) 5!

d) 1!

e) 3! + 2!

f)

*Solution: *

a) 3! = 3 × 2 × 1 = 6

b) 0! = 1

c) 5! = 5 × 4 × 3 × 2 × 1 = 120

d) 1! = 1

e) 3! + 2! = (3 × 2 × 1) + (2 × 1) = 8

f)

The following video gives same basic information on factorials and shows how to evaluate some factorial examples.

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