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0606 P21 - Jun 2025 - Q2 - 5 marks
7140
In this question, \(k\) is a constant. It is given that \(2 x^{2}+(3 k-2) x+k=0\) has roots that are real and distinct. Find the set of possible values of \(k\).
Solution
Answer: \(k\lt \frac{2}{9}\) or \(k\gt 2\).
For a quadratic equation to have real and distinct roots, its discriminant must be positive.
The equation is
\(2x^2+(3k-2)x+k=0.\)
Here
\(a=2,\quad b=3k-2,\quad c=k.\)
So
\((3k-2)^2-4(2)(k)\gt 0.\)
Expand and simplify:
\(9k^2-12k+4-8k\gt 0.\)
Therefore
\(9k^2-20k+4\gt 0.\)
Factorise:
\((9k-2)(k-2)\gt 0.\)
The critical values are
\(k=\frac29\quad\text{and}\quad k=2.\)
The quadratic expression in \(k\) has positive leading coefficient, so it is positive outside the two roots.