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Let q = a / b and r = c / d be two rational numbers written in lowest terms. Let s = q + r and s = e / f be written in lowest terms. Assume that is not 0. Prove or disprove the following two statements. A.) If b and d are odd, then f is odd. B.) If b and d are even, then f is even.

Respuesta :

Both the statements are true.

  • q = a/b and r = c/d.
  • s = q + r
  • s = a/b + c/d
  • s = (ad + bc)/bd
  • e/f = (ad + bc)/bd
  • e = ad + bc and f = bd
  • a) If b and d are odd, we know that the product of two odd numbers is always odd, so f = b*d is odd.
  • We can prove this because b*d = b*(d-1+1)
  • b*(d-1+1) = b*(d-1) + b
  • Since d is odd, d-1 is even, and therefore b*(d-1) is even. Since b is odd and the sum of an odd number and an even number is always odd, the above product is odd.
  • b) If b and d are even, we know that the product of two even numbers is even, and therefore f = b*d is even.

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