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Miscellaneous Exercise · Q3

Q.−3≤4−7x2≤18-3 \le 4 - \dfrac{7x}{2} \le 18

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This is a compound linear inequality. The key is to isolate xx by performing the same operations on all three parts, reversing the inequality sign when multiplying or dividing by a negative number. The solution is −4≤x≤2-4 \le x \le 2.

Understanding the Problem

We have a compound inequality: two inequality statements joined by "and". It says that the expression 4−7x24 - \frac{7x}{2} lies between −3-3 and 1818, inclusive. Our job is to find all values of xx that satisfy this.

The core idea is simple: treat the three parts (left, middle, right) like the two sides of a regular equation, but now we have three "sides". Whatever operation we do to the middle, we must do to all three parts to keep the inequalities balanced.

Watch out

A common mistake is forgetting to reverse the inequality sign when multiplying or dividing by a negative number. Here, the coefficient of xx is −72-\frac{7}{2}, which is negative — so when we isolate xx, we must flip both inequality signs.

Step-by-Step Solution

1. Write the compound inequality clearly

We have:

−3≤4−7x2≤18-3 \le 4 - \frac{7x}{2} \le 18

2. Subtract 4 from all three parts

We want to isolate the term containing xx. Subtracting 4 from each part:

−3−4≤4−7x2−4≤18−4-3 - 4 \le 4 - \frac{7x}{2} - 4 \le 18 - 4

This simplifies to:

−7≤−7x2≤14-7 \le -\frac{7x}{2} \le 14

3. Multiply all three parts by −1-1 to make the xx coefficient positive

Multiplying by a negative number reverses the inequality signs. So:

7≥7x2≥−147 \ge \frac{7x}{2} \ge -14

It's more conventional to write the smaller number on the left, so we can rewrite this as:

−14≤7x2≤7-14 \le \frac{7x}{2} \le 7

Tip

When you multiply an inequality by a negative number, think of it as "flipping the whole number line". The order reverses: what was on the left becomes on the right. Always double-check with a simple example like 1<21 < 2 becoming −1>−2-1 > -2.

4. Multiply all three parts by 27\frac{2}{7} to isolate xx

Since 72×27=1\frac{7}{2} \times \frac{2}{7} = 1, multiplying by 27\frac{2}{7} (a positive number) does not change the inequality direction:

−14×27≤x≤7×27-14 \times \frac{2}{7} \le x \le 7 \times \frac{2}{7}

Simplify:

−4≤x≤2-4 \le x \le 2

5. Interpret the result

xx can be any real number from −4-4 to 22, inclusive. This is a closed interval [−4,2][-4, 2].

Note

You can verify by picking a value inside, say x=0x=0: 4−0=44 - 0 = 4, which lies between −3-3 and 1818. Pick x=−4x=-4: 4−7(−4)2=4+14=184 - \frac{7(-4)}{2} = 4 + 14 = 18, the upper bound. Pick x=2x=2: 4−7(2)2=4−7=−34 - \frac{7(2)}{2} = 4 - 7 = -3, the lower bound. So the endpoints work.

✓Final answer

The solution set is −4≤x≤2-4 \le x \le 2, or in interval notation [−4,2][-4, 2].

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