The diagram below shows the curve y=4e2x−7y = 4e^{2x} - 7 and the curve y=2e−2xy = 2e^{-2x}.

  1. (a)

    Using the diagram above, determine with explanation, the number of solutions for the equation 4e2x−2e−2x=74e^{2x} - 2e^{-2x} = 7.

  2. (b)

    Solve the equation 4e2x−7=2e−2x4e^{2x} - 7 = 2e^{-2x}.

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Worked solution

1(a)

1. Rearrange the given equation to match the equations of the curves in the diagram

The equation 4e2x−2e−2x=74e^{2x} - 2e^{-2x} = 7 can be rearranged as:
4e2x−7=2e−2x4e^{2x} - 7 = 2e^{-2x}

2. Relate the equation to the intersection of the curves and state the number of solutions

The solutions to 4e2x−7=2e−2x4e^{2x} - 7 = 2e^{-2x} correspond to the points of intersection of the curves y=4e2x−7y = 4e^{2x} - 7 and y=2e−2xy = 2e^{-2x}.
From the diagram, the two curves intersect at exactly one point. Therefore, there is 1 solution.

Answer

1 solution

1(b)

1. Transform the equation into a quadratic equation in terms of $e^{2x}$

Let u=e2xu = e^{2x}. Then e−2x=1ue^{-2x} = \frac{1}{u}.
4u−7=2u4u - 7 = \frac{2}{u}
4u2−7u−2=04u^2 - 7u - 2 = 0

2. Solve the quadratic equation for $u$

(4u+1)(u−2)=0(4u + 1)(u - 2) = 0
u=−14oru=2u = -\frac{1}{4} \quad \text{or} \quad u = 2

3. Substitute back $e^{2x}$ and reject the invalid solution

e2x=−14ore2x=2e^{2x} = -\frac{1}{4} \quad \text{or} \quad e^{2x} = 2
Since e2x>0e^{2x} > 0 for all real xx, e2x=−14e^{2x} = -\frac{1}{4} has no real solution.

4. Solve for $x$ using natural logarithms

e2x=2e^{2x} = 2
2x=ln⁡22x = \ln 2
x=12ln⁡2≈0.347 (to 3 s.f.)x = \frac{1}{2}\ln 2 \approx 0.347 \text{ (to 3 s.f.)}

Answer

x=12ln⁡2x = \frac{1}{2}\ln 2 or x≈0.347x \approx 0.347

AI analysis — may contain mistakes.

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