Q.
This is a compound linear inequality. The key is to isolate by performing the same operations on all three parts, reversing the inequality sign when multiplying or dividing by a negative number. The solution is .
Understanding the Problem
We have a compound inequality: two inequality statements joined by "and". It says that the expression lies between and , inclusive. Our job is to find all values of 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.
A common mistake is forgetting to reverse the inequality sign when multiplying or dividing by a negative number. Here, the coefficient of is , which is negative — so when we isolate , we must flip both inequality signs.
Step-by-Step Solution
1. Write the compound inequality clearly
We have:
2. Subtract 4 from all three parts
We want to isolate the term containing . Subtracting 4 from each part:
This simplifies to:
3. Multiply all three parts by to make the coefficient positive
Multiplying by a negative number reverses the inequality signs. So:
It's more conventional to write the smaller number on the left, so we can rewrite this as:
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 becoming .
4. Multiply all three parts by to isolate
Since , multiplying by (a positive number) does not change the inequality direction:
Simplify:
5. Interpret the result
can be any real number from to , inclusive. This is a closed interval .
You can verify by picking a value inside, say : , which lies between and . Pick : , the upper bound. Pick : , the lower bound. So the endpoints work.
The solution set is , or in interval notation .
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