The function f is defined for x≠2x \ne 2 by f:x↦kx2−5x+3x−2f : x \mapsto \dfrac{kx^{2}-5x+3}{x-2}, where k is a real constant. Find the range of values of k such that the function is an increasing function for all real values of x,x≠2x, x \ne 2.

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Worked solution

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1. Find the derivative of the function $f(x)$ using the quotient rule.

f′(x)=ddx(kx2−5x+3x−2)f'(x) = \frac{\text{d}}{\text{d}x} \left( \frac{kx^2 - 5x + 3}{x - 2} \right)
Using the quotient rule:
f′(x)=(2kx−5)(x−2)−(kx2−5x+3)(1)(x−2)2f'(x) = \frac{(2kx - 5)(x - 2) - (kx^2 - 5x + 3)(1)}{(x - 2)^2}
f′(x)=2kx2−4kx−5x+10−kx2+5x−3(x−2)2f'(x) = \frac{2kx^2 - 4kx - 5x + 10 - kx^2 + 5x - 3}{(x - 2)^2}
f′(x)=kx2−4kx+7(x−2)2f'(x) = \frac{kx^2 - 4kx + 7}{(x - 2)^2}

2. State the condition for the function to be increasing.

For f(x)f(x) to be an increasing function for all x≠2x \neq 2, we require f′(x)≥0f'(x) \geq 0 for all x≠2x \neq 2.
kx2−4kx+7(x−2)2≥0\frac{kx^2 - 4kx + 7}{(x - 2)^2} \geq 0
Since (x−2)2>0(x - 2)^2 > 0 for all x≠2x \neq 2, this inequality holds if and only if the numerator is non-negative:
kx2−4kx+7≥0for all x≠2.kx^2 - 4kx + 7 \geq 0 \quad \text{for all } x \neq 2.

3. Formulate conditions for the numerator to be consistently non-negative.

If k=0k = 0, the numerator becomes 0x2−4(0)x+7=7≥00x^2 - 4(0)x + 7 = 7 \geq 0, which is valid for all real xx.
If k≠0k \neq 0, the quadratic expression must have a positive leading coefficient (k>0k > 0) and a non-positive discriminant (Δ≤0\Delta \leq 0) so that it is always non-negative:
Δ=(−4k)2−4(k)(7)≤0\Delta = (-4k)^2 - 4(k)(7) \leq 0
16k2−28k≤016k^2 - 28k \leq 0
4k(4k−7)≤04k(4k - 7) \leq 0

4. Solve the inequality to find the final range of $k$.

Solving 4k(4k−7)≤04k(4k - 7) \leq 0 yields:
0≤k≤740 \leq k \leq \frac{7}{4}
Combining this with the valid case k=0k = 0, the complete range of values for kk is:
0≤k≤740 \leq k \leq \frac{7}{4}

Answer

0≤k≤740 \leq k \leq \frac{7}{4}

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