Find the range of values of kk such that 3x2−5x+k3x^2 - 5x + k is always positive. Hence, solve the inequality x2−x−23x2−5x+4<0\frac{x^2 - x - 2}{3x^2 - 5x + 4} < 0.

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Worked solution

2(a)

1. State the condition for the quadratic expression to be always positive

For 3x2−5x+k>03x^2 - 5x + k > 0 for all real xx, since the coefficient of x2x^2 is 3>03 > 0, the discriminant must be less than zero.
b2−4ac<0b^2 - 4ac < 0

2. Substitute the coefficients into the discriminant and solve for $k$

(−5)2−4(3)(k)<0(-5)^2 - 4(3)(k) < 0
25−12k<025 - 12k < 0
12k>2512k > 25
k>2512k > \frac{25}{12}

Answer

k>2512k > \frac{25}{12}

2(b)

1. Deduce the sign of the denominator using the result from part (a)

From part (a), 3x2−5x+k>03x^2 - 5x + k > 0 for all real xx when k>2512k > \frac{25}{12}.
Since 4>25124 > \frac{25}{12}, the denominator 3x2−5x+43x^2 - 5x + 4 is always positive.

2. Simplify the inequality

Since 3x2−5x+4>03x^2 - 5x + 4 > 0, we can multiply both sides of the inequality by the denominator without changing the inequality sign:
x2−x−2<0x^2 - x - 2 < 0

3. Solve the quadratic inequality

(x−2)(x+1)<0(x - 2)(x + 1) < 0
−1<x<2-1 < x < 2

Answer

−1<x<2-1 < x < 2

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