[Cambridge/9709/2007] Find the exact value of the constant kk for which ∫1k12x−1 dx=1\int_{1}^{k} \frac{1}{2x - 1} \, dx = 1.

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Worked solution

Q1

1. Find the indefinite integral of the given function.

∫12x−1 dx=12ln⁡(2x−1)\int \frac{1}{2x - 1} \, dx = \frac{1}{2}\ln(2x - 1)

2. Evaluate the definite integral using the upper limit $k$ and lower limit $1$.

[12ln⁡(2x−1)]1k=1\left[ \frac{1}{2}\ln(2x - 1) \right]_1^k = 1
12ln⁡(2k−1)−12ln⁡(2(1)−1)=1\frac{1}{2}\ln(2k - 1) - \frac{1}{2}\ln(2(1) - 1) = 1

3. Simplify the equation using the value of $\ln(1)$.

Since ln⁡(1)=0\ln(1) = 0, the equation simplifies to:
12ln⁡(2k−1)−0=1\frac{1}{2}\ln(2k - 1) - 0 = 1
12ln⁡(2k−1)=1\frac{1}{2}\ln(2k - 1) = 1

4. Solve the logarithmic equation to find the exact value of $k$.

ln⁡(2k−1)=2\ln(2k - 1) = 2
2k−1=e22k - 1 = e^2
2k=e2+12k = e^2 + 1
k=12(e2+1)k = \frac{1}{2}(e^2 + 1)

Answer

k=12(e2+1)k = \frac{1}{2}(e^2 + 1)

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