The functions f and g are defined for all real values of x by
\(f(x) = 2x^2 - 3x\) and \(g(x) = 3x + k\),
where \(k\) is a constant.
Find the value of \(k\) for which the equation \(gf(x) = 0\) has equal roots.
Solution
To find the value of \(k\) for which \(gf(x) = 0\) has equal roots, we first find \(gf(x)\).
\(gf(x) = g(f(x)) = g(2x^2 - 3x) = 3(2x^2 - 3x) + k = 6x^2 - 9x + k\).
For the quadratic equation \(6x^2 - 9x + k = 0\) to have equal roots, the discriminant must be zero.
The discriminant \(b^2 - 4ac\) for the quadratic \(ax^2 + bx + c = 0\) is given by:
\(b^2 - 4ac = (-9)^2 - 4(6)(k) = 81 - 24k\).
Setting the discriminant to zero for equal roots:
\(81 - 24k = 0\).
Solving for \(k\):
\(24k = 81\)
\(k = \frac{81}{24} = \frac{27}{8}\).
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