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Question
Evaluate `("n"!)/("r"!("n" - "r")!)` when for any n with r = 2
Solution
For any n with r = 2
`("n"!)/("r"!("n" - "r")!)= ("n"!)/(2!("n" - 2)!)`
= `("n"("n" - 1)("n" - 2)!)/(1 xx 2 xx ("n" - 2)1)`
= `("n"("n" - 1))/2`
= `("n"^2 - "n")/2`
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