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Miscellaneous Examples · Example 10

Q.Solve −5≤5−3x2≤8-5 \le \dfrac{5 - 3x}{2} \le 8.

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✓ Free question

This is a compound linear inequality. The key is to isolate xx by performing the same operations on all three parts, reversing the inequality signs when multiplying or dividing by a negative number. The solution is −113≤x≤5-\frac{11}{3} \le x \le 5.

The Concept: What Does This Mean?

When you see an inequality like −5≤5−3x2≤8-5 \le \frac{5-3x}{2} \le 8, it's actually two inequalities in one:

  1. −5≤5−3x2-5 \le \dfrac{5-3x}{2}
  2. 5−3x2≤8\dfrac{5-3x}{2} \le 8

And we need the values of xx that satisfy both simultaneously. The neat trick is that we can work on all three parts at once — the left, middle, and right — as long as we do the same operation to each.

The only special rule? If we multiply or divide by a negative number, the inequality signs flip direction. That's the one thing students forget most often.


Step-by-Step Solution

1. Clear the denominator first

The fraction 5−3x2\dfrac{5-3x}{2} is sandwiched between −5-5 and 88. To remove the denominator, multiply all three parts by 22 (a positive number, so the inequality signs stay as they are):

−5×2≤5−3x2×2≤8×2-5 \times 2 \le \frac{5-3x}{2} \times 2 \le 8 \times 2

−10≤5−3x≤16-10 \le 5 - 3x \le 16

Now we have a simpler compound inequality with no fractions.

2. Isolate the term containing xx

We want xx alone, so first remove the +5+5 from the middle. Subtract 55 from all three parts:

−10−5≤5−3x−5≤16−5-10 - 5 \le 5 - 3x - 5 \le 16 - 5

−15≤−3x≤11-15 \le -3x \le 11

Watch out

At this stage, many students rush to divide by −3-3 but forget to flip the inequality signs. The middle term is −3x-3x, and dividing by a negative number reverses the direction of both inequalities.

3. Divide by −3-3 and flip the signs

Divide every part by −3-3. Since −3-3 is negative, the ≤\le signs become ≥\ge:

−15−3≥−3x−3≥11−3\frac{-15}{-3} \ge \frac{-3x}{-3} \ge \frac{11}{-3}

Simplifying:

5≥x≥−1135 \ge x \ge -\frac{11}{3}

4. Rewrite in standard order

It's conventional to write inequalities with the smaller number on the left. So flip the order (but keep the signs pointing the same way):

−113≤x≤5-\frac{11}{3} \le x \le 5

Tip

A quick check: pick a value inside this range, say x=0x = 0. Then 5−02=2.5\frac{5-0}{2} = 2.5, which is indeed between −5-5 and 88. Pick x=6x = 6 (outside the range): 5−182=−6.5\frac{5-18}{2} = -6.5, which is less than −5-5 — so it fails. The boundaries work too.


✓Final answer

The solution is −113≤x≤5-\frac{11}{3} \le x \le 5, or in interval notation [−113, 5]\left[-\frac{11}{3},\ 5\right].

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