2.(i) We prove first the direct implication. Assume is even. Then for some integer Then which is even.
For the converse, which is “if is even, then is even”, we use a proof by contrapositive.
The contrapositive is: “if is not even (that is, odd), then is not even (that is, odd)”. If is odd, then for some integer
Then which is odd.
Hence we have proven that is even if and only if is even.
(ii) We will use a direct proof on “If is odd, then is odd”. Assume is odd, so for some integer
Then
Thus, is odd.
We now must prove the converse, “If is odd, then is odd.” For this, we will use proof by contrapositive. So the statement becomes “If is not odd, then is not odd.” Assume that is not odd, so for some integer
Then
Thus, is not odd.
Hence we have proven that is odd if and only if is odd.
(iii) If then follows using basic principles of Algebra.
Similarly, if then follows using basic principles of Algebra.
Suppose that 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: or Thus, using basic principles of algebra, either or
(iv)
Case 1: Suppose Regardless of what is, so But we said that both and are integers. If then here cannot be an integer. So, contradiction.
Case 2: Suppose Regardless of what is, so But we said that both and are integers. If then here cannot be an integer. So, contradiction.
Case 3: Suppose and Clearly then So, contradiction.
Case 4: Suppose and Clearly then So, contradiction.
Case 5: Suppose and Then
Case 6: Suppose and Then
What techniques did we use? Cases and contradiction
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