These Year 13 proof by contradiction multiple choice questions check your understanding of proof by contradiction with 10 questions for pupils aged 17-18. Each question has four answers, and the right one is explained as soon as you choose.
Year 13 proof by contradiction multiple choice questions: the 10 questions
What is the first line of a proof by contradiction that \(\sqrt{5}\) is irrational?
You assume the opposite of what you want to prove: that \(\sqrt5\) is rational.
The negation of "all prime numbers are odd" is
Negating "for all" gives "there exists" with the property reversed.
In the proof that \(\sqrt2\) is irrational, the contradiction is that
Both a and b have factor 2.
Which statement is a valid counterexample to "if \(n\) is prime then \(n+2\) is prime"?
7 is prime but 9 is not. For 3, 5 and 11, n+2 is 5, 7 and 13.
If \(N=2\times3\times5\times7\times11\times13+1=30031\), then
30031 = 59 x 509; neither of these is in the list.
Which of these cannot be proved by giving examples?
Examples cannot prove a "for all" claim.
If \(x\) is irrational, then \(x+\tfrac12\) is
Rational plus irrational is irrational.
Which is the correct negation of "\(n^2\) is even"?
The opposite of even is odd (for the same quantity, \(n^2\)).
Prove \(14m+21n=1\) has no integer solutions: the key fact is that
7(2m+3n) cannot equal 1.
In the proof of infinitely many primes, which fact leads to the contradiction?
The difference N minus the product is 1.
How to use these Year 13 proof by contradiction multiple choice questions
First, read each question carefully before you look at the four answers. Then work the answer out on paper, so that you do not simply guess. After that, read the explanation, even when you are right. If you score less than 7 out of 10, go back to the proof by contradiction lesson.
Finally, practise more with the proof by contradiction worksheet and the quiz. Moreover, all the pages of this chapter work together, so you can move between them at any time. For example, many pupils read the lesson on Monday, practise on Wednesday and test themselves on Friday.