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mliuzzolino
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Homework Statement
Prove that if n is a natural number greater than 1, then n-1 is also a natural number. (Hint: Prove that the set {n | n = 1 or n in [itex] \mathbb{N} [/itex] and n - 1 in [itex] \mathbb{N} [/itex]} is inductive.)
Homework Equations
The Attempt at a Solution
[itex] S(n) = \sum_{j = 2}^{n} j = 2 + 3 + \cdots + n = \dfrac{(n-1)(n+2)}{2} [/itex]
Checking the induction hypothesis, S(2) = 2 = (1)(4)/(2) = 2. True.
Suppose the statement [itex] P(k): 2 + 3 + \cdots + k = \dfrac{(k-1)(k+2)}{2} [/itex] is true. Then let P(k-1) be the statement:
[itex] 2 + 3 + \cdots + (k - 1) [/itex]
We want to show that this equals [itex] \dfrac{(k-2)(k+1)}{2} [/itex].
[itex] 2 + 3 + \cdots + (k - 1) = (2 + 3 + \cdots + k) - 1 = \dfrac{(k-1)(k+2)}{2} - 1
= \dfrac{k^2 + k - 2 - 2}{2} = \dfrac{k^2 +k -4}{2}.
[/itex]I'm not sure where I'm going wrong with this, but I'm feeling quite lost at this point. I'm self studying using "Advanced Calculus by Fitzpatrick," and my university does not have upper division math tutoring over the summer. I greatly appreciate any points in the right direction!
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