Suppose x > 3 and y < 2. Then x² − 2y > 5.
Let’s break this down step by step.
We’re told:
- $x > 3$
- $y < 2$
We want to see if these imply:
$$ x^2 - 2y > 5 $$
Step 1: Lower bound for $x^2$
If $x > 3$, then:
$$ x^2 > 9 $$
Step 2: Lower bound for $-2y$
If $y < 2$, then multiplying both sides by $-2$ (which reverses the inequality):
$$ -2y > -4 $$
So the smallest possible value for $-2y$ is just greater than $-4$. It can be much larger if $y$ is negative (or even very large if $y$ is very negative).
Step 3: Combine
We have:
$$ x^2 - 2y > 9 - 4 = 5 $$
That’s already enough to conclude $x^2 - 2y > 5$, because:
- $x^2$ is strictly greater than 9,
- $-2y$ is strictly greater than $-4$, so their sum is strictly greater than $9 - 4 = 5$.
✅ Conclusion: Yes — from $x > 3$ and $y < 2$, it follows that:
$$ x^2 - 2y > 5 $$
If you’d like, I can also show you the geometric interpretation of this inequality in the $(x, y)$-plane, so you can see exactly which region these conditions describe.
No comments:
Post a Comment