Question #123219
2. (i) Prove that if n is a positive integer, then n is even if and only if 7n + 4 is even.
(ii) Prove that if n is a positive integer, then n is odd if and only if 5n + 6 is odd.
(iii) Prove that m2 = n2 if and only if m = n or m = -n.
(iv) Prove or disprove that if m and n are integers such that mn = 1, then either m = 1 or else m = -1 and n = -1.
1
Expert's answer
2020-06-22T17:36:33-0400

2.(i)=>=> We prove first the direct implication. Assume nn is even. Then n=2kn=2k for some integer k.k. Then 7n+4=7(2k)+4=14k+4=2(7k+2),7n+4=7(2k)+4=14k+4=2(7k+2), which is even.


<=<= For the converse, which is “if 7n+47n+4 is even, then nn is even”, we use a proof by contrapositive.

The contrapositive is: “if nn is not even (that is, odd), then 7n+47n+4 is not even (that is, odd)”. If nn is odd, then n=2k+1,n=2k+1, for some integer k.k.

Then 7n+4=7(2k+1)+4=14k+7+4=14k+11,7n+4=7(2k+1)+4=14k+7+4=14k+11, which is odd.

Hence we have proven that nn is even if and only if 7n+47n+4 is even. 


(ii) =>=> We will use a direct proof on “If nn is odd, then 5n+65n+6 is odd”. Assume nn is odd, so n=2k+1n=2k+1 for some integer k.k.

Then 5n+6=5(2k+1)+6=10k+5+65n+6=5(2k+1)+6=10k+5+6 =10k+11=2(5k+5)+1.=10k+11=2(5k+5)+1.

Thus, 5n+65n+6 is odd.


<=<= We now must prove the converse, “If 5n+65n+6 is odd, then nn is odd.” For this, we will use proof by contrapositive. So the statement becomes “If nn is not odd, then 5n+65n+6 is not odd.” Assume that nn is not odd, so n=2kn=2k for some integer k.k.

Then 5n+6=5(2k)+6=10k+6=2(5k+3).5n+6=5(2k)+6=10k+6=2(5k+3).

Thus, 5n+65n+6 is not odd.

Hence we have proven that nn is odd if and only if 5n+65n+6 is odd. 


(iii) =>=> If m=n,m=n, then m2=n2m^2=n^2 follows using basic principles of Algebra.

Similarly, if m=n,m=-n, then m2=(n)2=n2m^2=(-n)^2=n^2 follows using basic principles of Algebra.

<=<= Suppose that m2=n2.m^2=n^2. Notice that the square of any real number is non-negative, so we can take the square root of each side of this equation in order to obtain: m2=n2,\sqrt{m^2}=\sqrt{n^2}, or m=n.|m|=|n|. Thus, using basic principles of algebra, either m=nm=n or m=n.m=-n.


(iv) mn=1=>m0,n0.mn=1=>m\not=0, n\not=0.

Case 1: Suppose m>1.|m|>1. Regardless of what nn is, mn=1,mn=1, so n=1m.n=\dfrac{1}{m}. But we said that both mm and nn are integers. If m>1,|m|>1, then nn here cannot be an integer. So, contradiction.

Case 2: Suppose n>1.|n|>1. Regardless of what mm is, mn=1,mn=1, so n=1m.n=\dfrac{1}{m}. But we said that both mm and nn are integers. If n>1,|n|>1, then mm here cannot be an integer. So, contradiction.

Case 3: Suppose m=1m=1 and n=1.n=-1. Clearly then mn=1.mn=-1. So, contradiction.

Case 4: Suppose m=1m=-1 and n=1.n=1. Clearly then mn=1.mn=-1. So, contradiction.

Case 5: Suppose m=1m=1 and n=1.n=1. Then mn=1.mn=1.

Case 6: Suppose m=1m=-1 and n=1.n=-1. Then mn=1.mn=1.

What techniques did we use? Cases and contradiction



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