To prove something by contradiction, we assume that what we want to prove is not true, and then show that the consequences of this are not possible. That is, the consequences contradict either what we have just assumed, or something we already know to be true (or, indeed, both) - we call this a contradiction
The LCHL course requires that students can give an example of proof by contradiction. The example below is as easy as it gets.
Discussed Interquartile range.
Read through Section 2.8 looking at Skew Curves/Histograms.
HW: End of Chapter 2 questions.