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Miscellaneous Exercise · Q6

Q.7≤3x+112≤117 \le \dfrac{3x + 11}{2} \le 11

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The key idea is to treat the compound inequality as two separate inequalities and solve for xx by isolating it through algebraic operations. The solution set is 1≤x≤1131 \le x \le \frac{11}{3}, or x∈[1,113]x \in \left[1, \frac{11}{3}\right].

Concept and Intuition

A compound inequality like 7≤3x+112≤117 \le \frac{3x + 11}{2} \le 11 means that the expression 3x+112\frac{3x + 11}{2} lies between 7 and 11, inclusive. To find the range of xx, we need to "undo" the operations applied to xx — first the division by 2, then the addition of 11, and finally the multiplication by 3 — but we must do this to all three parts of the inequality simultaneously. This preserves the relationship: whatever we do to the middle, we do to the left and right ends.

The core principle: linear inequalities behave like equations when adding, subtracting, multiplying, or dividing by a positive number. The only twist is that multiplying or dividing by a negative number flips the inequality signs — but here, all operations involve positive numbers, so no flipping occurs.

Watch out

A common mistake is to multiply only the middle term by 2, forgetting to multiply the 7 and 11 as well. Always apply the same operation to all three parts of a compound inequality.

Step-by-Step Solution

  1. Write the compound inequality clearly We have:

7≤3x+112≤117 \le \frac{3x + 11}{2} \le 11

  1. Eliminate the denominator Multiply every part of the inequality by 2 (a positive number, so inequality signs stay the same):

7×2≤3x+112×2≤11×27 \times 2 \le \frac{3x + 11}{2} \times 2 \le 11 \times 2

This simplifies to:

14≤3x+11≤2214 \le 3x + 11 \le 22

  1. Isolate the term with xx Subtract 11 from all three parts (again, no sign change):

14−11≤3x+11−11≤22−1114 - 11 \le 3x + 11 - 11 \le 22 - 11

Which gives:

3≤3x≤113 \le 3x \le 11

  1. Solve for xx …

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