Mathematical Proofs And Induction: Set Theory, Even/Odd Properties, And Sum Of Cubes

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Mathematical Proofs and Induction: Set Theory, Even/Odd Properties,and Sum of CubesJeremy BreitMarch 25, 2013MTH 231Homework #2Problem:Prove that, for all integers n, if n2+ 3 is even, then n is odd.Solution:1. Assuming n2+ 3 is even, we can say that 2 divides the equation. n2+ 32|(𝑛2+3)βŸΊπ‘›2+3=2π‘˜π‘“π‘œπ‘Ÿπ‘ π‘œπ‘šπ‘’π‘˜πœ–β„€Simply,𝑛2+3is some integer multiple of 2.2. Since 2(k-2) is divisible by 2, and 2 is not divisible by 1, we can say thefollowing:𝑛2=2π‘˜βˆ’3=2(π‘˜βˆ’2)+1⟹2π‘‘π‘œπ‘’π‘ π‘›π‘œπ‘‘π‘‘π‘–π‘£π‘–π‘‘π‘’2(π‘˜βˆ’2)+1,⟹2π‘‘π‘œπ‘’π‘ π‘›π‘œπ‘‘π‘‘π‘–π‘£π‘–π‘‘π‘’π‘›2Simply, since 2 does not divide𝑛2,𝑛2cannot be even. This means that𝑛2is odd.3. Next we need to prove that since𝑛2is odd,𝑛is therefor odd. To do this, wewill prove the contrapositive. In this case, we will assume𝑛is even.𝑛=2π‘˜π‘“π‘œπ‘Ÿπ‘ π‘œπ‘šπ‘’π‘–π‘›π‘‘π‘’π‘”π‘’π‘Ÿπ‘˜Therefor,𝑛2=(2π‘˜)2=2(2π‘˜2)4. Since 2 divides𝑛2,𝑛2must be even. Therefor, is n is not odd,𝑛2is not odd aswell.5. Contrapositively, since𝑛2is odd,𝒏must be odd as well.

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