A curve is defined by the equation \(y = 2x^2 - 3x\). Determine the set of \(x\) values for which \(y > 9\).
Solution
To find the set of \(x\) values for which \(y > 9\), start with the inequality:
\(2x^2 - 3x > 9\)
Subtract 9 from both sides:
\(2x^2 - 3x - 9 > 0\)
Factor the quadratic expression:
\((2x + 3)(x - 3) > 0\)
Find the roots by setting each factor to zero:
\(2x + 3 = 0 \Rightarrow x = -\frac{3}{2}\)
\(x - 3 = 0 \Rightarrow x = 3\)
Test intervals around the roots to determine where the inequality holds:
- For \(x < -\frac{3}{2}\), choose \(x = -2\): \((2(-2) + 3)((-2) - 3) = (-1)(-5) = 5 > 0\)
- For \(-\frac{3}{2} < x < 3\), choose \(x = 0\): \((2(0) + 3)(0 - 3) = (3)(-3) = -9 < 0\)
- For \(x > 3\), choose \(x = 4\): \((2(4) + 3)(4 - 3) = (11)(1) = 11 > 0\)
The solution is \(x > 3\) or \(x < -\frac{3}{2}\).
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