Express 33−8xx2+2x−15+2\dfrac{33-8x}{x^2+2x-15}+2 as a single algebraic fraction. Hence, without using a calculator, solve exactly the inequality 33−8xx2+2x−15>−2\dfrac{33-8x}{x^2+2x-15}>-2. Using your answer to part (a), find the set of values of xx for which 33−8e2xe4x+2e2x−15>−2\dfrac{33-8e^{2x}}{e^{4x}+2e^{2x}-15}>-2.

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Worked solution

1(a)

1. Combine the expression into a single algebraic fraction.

33−8xx2+2x−15+2\frac{33-8x}{x^2+2x-15}+2
=33−8x+2(x2+2x−15)x2+2x−15= \frac{33-8x+2(x^2+2x-15)}{x^2+2x-15}
=33−8x+2x2+4x−30x2+2x−15= \frac{33-8x+2x^2+4x-30}{x^2+2x-15}
=2x2−4x+3x2+2x−15= \frac{2x^2-4x+3}{x^2+2x-15}

2. Rewrite the inequality to compare with zero and analyse the sign of the numerator.

33−8xx2+2x−15>−2\frac{33-8x}{x^2+2x-15} > -2
33−8xx2+2x−15+2>0\frac{33-8x}{x^2+2x-15} + 2 > 0
2x2−4x+3x2+2x−15>0\frac{2x^2-4x+3}{x^2+2x-15} > 0
Check the discriminant (Δ\Delta) of the numerator 2x2−4x+32x^2-4x+3:
Δ=(−4)2−4(2)(3)=16−24=−8\Delta = (-4)^2 - 4(2)(3) = 16 - 24 = -8
Since the discriminant is negative (−8<0-8 < 0) and the coefficient of x2x^2 is positive (2>02 > 0), the quadratic expression is always positive, meaning 2x2−4x+3>02x^2-4x+3 > 0 for all real xx.

3. Determine the required condition for the denominator and solve the resulting inequality.

For the rational fraction to be positive when the numerator is strictly positive, the denominator must also be strictly positive:
x2+2x−15>0x^2+2x-15 > 0
Factorising the quadratic expression:
(x+5)(x−3)>0(x+5)(x-3) > 0
Solving this inequality gives:
x<−5 or x>3x < -5 \text{ or } x > 3

Answer

Single fraction: 2x2−4x+3x2+2x−15\dfrac{2x^2-4x+3}{x^2+2x-15}

Solution: x<−5 or x>3x < -5 \text{ or } x > 3

1(b)

1. Apply the result from part (a) by using the substitution $x = e^{2x}$.

From part (a), solving 33−8xx2+2x−15>−2\dfrac{33-8x}{x^2+2x-15} > -2 yields the solution x<−5x < -5 or x>3x > 3.

Replacing xx with e2xe^{2x} in the inequality gives:
e2x<−5ore2x>3e^{2x} < -5 \quad \text{or} \quad e^{2x} > 3

2. Solve the resulting exponential inequalities and reject any invalid cases.

Since the exponential function e2x>0e^{2x} > 0 for all real xx, the inequality e2x<−5e^{2x} < -5 has no real solution.

Solving the valid inequality:
e2x>3e^{2x} > 3
Taking the natural logarithm on both sides:
2x>ln⁡32x > \ln 3
x>12ln⁡3x > \frac{1}{2}\ln 3

Answer

x>12ln⁡3x > \frac{1}{2}\ln 3

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