This is a homework question from a precalculus class that I'm a TA for.

Graph on a real line all points $x$ such that \begin{equation}\tag{$\star$} (4x -2 \leq 6 \;\text{ and }\; 3-x \leq 7) \;\;\text{ or }\;\; (3x > 18 \;\text{ and }\; 2x-9 \geq 11)\,. \end{equation}

Enough students were struggling with it, especially with how to mathematically interpret "and" and "or", that I wanted to write up a thorough solution to this exercise for my class, and figured I'd post it online to help anyone else who may wander across it.


closed as off-topic by Carl Mummert, José Carlos Santos, Scientifica, Namaste, Don Thousand Nov 10 '18 at 21:43

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  • $\begingroup$ Mike, while I agree with the premise that some self answered questions can be of use, I'm not so sure about this one. This seems like a question very easily answered if one reads any of the questions on graphing inequalities. $\endgroup$ – Don Thousand Nov 10 '18 at 21:44
  • $\begingroup$ @RushabhMehta Then shouldn't this question be closed as a duplicate as one of those? It makes no sense to close for missing context. The context is clear and provided. $\endgroup$ – Mike Pierce Nov 10 '18 at 22:55

The four inequalities above can be written more simply as the following four inequalities: \begin{equation}\tag{$\star$} (x \leq 2 \;\text{ and }\; x \geq -4) \;\;\text{ or }\;\; (x > 6 \;\text{ and }\; x \geq 10) \end{equation} The word "and" between two inequalities indicates that we need them to simultaneously be true, which corresponds to taking the intersection of their individual solution sets. So the first pair of inequalities are true for all $x$ such that $-4 \leq x \leq 2$. On a number line, this looks like:

number line -4 \leq x \leq 2

Similarly for the other pair of inequalities, the intersection of their solution sets are all $x$ such that $x \geq 10$.

number line for x \geq 10

Now the word "or" between two inequalities indicates that we just need either one of them to be true, which corresponds to taking the union of their solution sets. So all the points $x$ that satisfy $(\star)$, when graphed on a number line, would look like

final number line, the union of the previous solution sets

  • $\begingroup$ Off-topic, but what do you use to make your number lines? $\endgroup$ – Toby Bartels Nov 6 '18 at 22:19
  • 1
    $\begingroup$ @TobyBartels I used tikzpicture in LaTeX. See this overleaf document for the source. :) $\endgroup$ – Mike Pierce Nov 6 '18 at 22:27
  • $\begingroup$ Thanks, I've been meaning to learn TikZ. $\endgroup$ – Toby Bartels Nov 6 '18 at 22:59

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