Meden Agan
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- Homework Statement
- Prove $$\int\limits_0^{\sqrt2/4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx = \frac{\pi^2}{8}.$$
- Relevant Equations
- Fubini's Theorem, arcsin representation integral.
Prove $$\int\limits_0^{\sqrt2/4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx = \frac{\pi^2}{8}.$$
Let $$I = \int\limits_0^{\sqrt 2 / 4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx. \tag{1}$$
The representation integral of ##\arcsin## is $$\arcsin u = \int\limits_{0}^{1} \frac{\mathrm dt}{\sqrt{1-t^2}}, \qquad 0 \leqslant u \leqslant 1.$$
Plugging identity above into ##(1)## with ##u = \sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}}##, we obtain
$$I = \int\limits_0^{\frac{\sqrt{2}}{4}} \frac{1}{\sqrt{x - x^2}} \int_0^{\sqrt{\frac{\left(x - 1\right) \left( x - 1 + \sqrt{9 - 16x} \right)}{1-2x}}} \frac{\mathrm dt}{\sqrt{1 - t^2}} \, \mathrm dx.$$
Since the integrand is non-negative and continuous over the rectangular domain ##D := \left\{(x, t) : 0 \leq x \leq \frac{\sqrt{2}}{4}, \,\, 0 \leq t \leq f(x)\right\}## (##f(x)## is the root of the numerator), Fubini's Theorem allows us to interchange the order:
$$I = \int\limits_0^1 \frac{\mathrm dt}{\sqrt{1 - t^2}} \int\limits_{x_*(t)}^{x^*(t)} \frac{\mathrm dx}{\sqrt{x- x^2}},$$
where ##x_*(t)## and ##x^*(t)## are the closed solutions of the equation
$$t^2 = \frac{(x - 1)\left(x - 1 + x \sqrt{9 - 16x}\right)}{1-2x}. \tag{*}$$
Now, computing the closed-form solutions of Equation ##(*)## looks like a lot of work. And even WolframAlpha returns a tremendous expression.
I'd like your help to get on with that integral.
Let $$I = \int\limits_0^{\sqrt 2 / 4}\frac{1}{\sqrt{x-x^2}}\arcsin\sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}} \, \mathrm dx. \tag{1}$$
The representation integral of ##\arcsin## is $$\arcsin u = \int\limits_{0}^{1} \frac{\mathrm dt}{\sqrt{1-t^2}}, \qquad 0 \leqslant u \leqslant 1.$$
Plugging identity above into ##(1)## with ##u = \sqrt{\frac{(x-1)\left(x-1+x\sqrt{9-16x}\right)}{1-2x}}##, we obtain
$$I = \int\limits_0^{\frac{\sqrt{2}}{4}} \frac{1}{\sqrt{x - x^2}} \int_0^{\sqrt{\frac{\left(x - 1\right) \left( x - 1 + \sqrt{9 - 16x} \right)}{1-2x}}} \frac{\mathrm dt}{\sqrt{1 - t^2}} \, \mathrm dx.$$
Since the integrand is non-negative and continuous over the rectangular domain ##D := \left\{(x, t) : 0 \leq x \leq \frac{\sqrt{2}}{4}, \,\, 0 \leq t \leq f(x)\right\}## (##f(x)## is the root of the numerator), Fubini's Theorem allows us to interchange the order:
$$I = \int\limits_0^1 \frac{\mathrm dt}{\sqrt{1 - t^2}} \int\limits_{x_*(t)}^{x^*(t)} \frac{\mathrm dx}{\sqrt{x- x^2}},$$
where ##x_*(t)## and ##x^*(t)## are the closed solutions of the equation
$$t^2 = \frac{(x - 1)\left(x - 1 + x \sqrt{9 - 16x}\right)}{1-2x}. \tag{*}$$
Now, computing the closed-form solutions of Equation ##(*)## looks like a lot of work. And even WolframAlpha returns a tremendous expression.
I'd like your help to get on with that integral.
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