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Worked solution

Determine a parameter from two exact integrals

Find the exact value of pp such that ∫01dx4−x2=∫01/(2p)dx1−p2x2\displaystyle\int_0^1\frac{dx}{4-x^2}=\int_0^{1/(2p)}\frac{dx}{\sqrt{1-p^2x^2}}.

CJC · Discussion · Question 3

Answer

p=2π/(3ln⁡3)p=2\pi/(3\ln3).

Evaluate the left-hand side and determine the sign of p

∫01dx/(4−x2)=[14ln⁡∣(2+x)/(2−x)∣]01=14ln⁡3>0\int_0^1dx/(4-x^2)=[\tfrac14\ln|(2+x)/(2-x)|]_0^1=\tfrac14\ln3>0. The right integral is undefined for p=0p=0 and negative for p<0p<0 because its upper limit is then negative while its integrand is positive. Therefore any solution has p>0p>0.

How did this part go?

Use the inverse-sine antiderivative

For p>0p>0, the right integral is [sin⁡−1(px)/p]01/(2p)=π/(6p)[\sin^{-1}(px)/p]_0^{1/(2p)}=\pi/(6p). Equating gives 14ln⁡3=π/(6p)\tfrac14\ln3=\pi/(6p), hence p=2π/(3ln⁡3)p=2\pi/(3\ln3).

How did this part go?

Common mistakes

Check the sign of the variable upper limit before assuming p is positive.

Read the hint