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Exercise 9.4 · Q3

Q.∫01/2esin⁡−1xsin⁡−1x1−x2 dx\displaystyle\int_0^{1/\sqrt2} \dfrac{e^{\sin^{-1}x}\sin^{-1}x}{\sqrt{1-x^2}}\,dx

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As in the previous problem, sin⁡−1x\sin^{-1}x is not a polynomial, so Bernoulli's formula cannot be applied to the original xx-integral directly. Substituting θ=sin⁡−1x\theta=\sin^{-1}x turns the sin⁡−1x\sin^{-1}x factor into a genuine linear polynomial in θ\theta.

Step 1. Substitute θ=sin⁡−1x\theta=\sin^{-1}x. Then dθ=dx1−x2d\theta=\dfrac{dx}{\sqrt{1-x^2}}, and the limits become x=0⇒θ=0x=0\Rightarrow\theta=0, x=1/2⇒θ=π/4x=1/\sqrt2\Rightarrow\theta=\pi/4. Since sin⁡−1x1−x2dx=θ dθ\dfrac{\sin^{-1}x}{\sqrt{1-x^2}}dx=\theta\,d\theta, the integral becomes

∫01/2esin⁡−1xsin⁡−1x1−x2dx=∫0π/4θeθ dθ.\int_0^{1/\sqrt2}\dfrac{e^{\sin^{-1}x}\sin^{-1}x}{\sqrt{1-x^2}}dx=\int_0^{\pi/4}\theta e^\theta\,d\theta.

Step 2. Recognize this is now a Bernoulli's-formula case. u=θu=\theta is a polynomial with u(1)=1, u(2)=0u^{(1)}=1,\ u^{(2)}=0, and v=eθv=e^\theta is repeatedly integrable.

Step 3. List the successive anti-derivatives of v=eθv=e^\theta. v(1)=∫eθ dθ=eθv_{(1)}=\displaystyle\int e^\theta\,d\theta=e^\theta, v(2)=∫v(1) dθ=eθv_{(2)}=\displaystyle\int v_{(1)}\,d\theta=e^\theta.

Step 4. Apply Bernoulli's formula. ∫uv dθ=uv(1)−u(1)v(2)\int uv\,d\theta=uv_{(1)}-u^{(1)}v_{(2)} (terminates since u(2)=0u^{(2)}=0):

∫θeθ dθ=θeθ−1⋅eθ=eθ(θ−1)=F(θ).\int\theta e^\theta\,d\theta=\theta e^\theta-1\cdot e^\theta=e^\theta(\theta-1)=F(\theta). …

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