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Q.Prove that ∫x2−a2 dx=12[xx2−a2−a2log⁡∣x+x2−a2∣]+C\displaystyle\int \sqrt{x^2-a^2}\,dx = \dfrac{1}{2}\left[x\sqrt{x^2-a^2} - a^2\log\left|x+\sqrt{x^2-a^2}\right|\right] + C OR State and prove the formula for integration by parts.

Manipur CohsemCOHSEM Manipur Higher Secondary Board 2025Subjective· 4mImportance★★★★★
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Part 1: Integrate by parts (treat x2−a2\sqrt{x^2-a^2} as the first function, 11 as the second), then solve the resulting equation for II. OR Part 2: State and prove the integration-by-parts formula using the product rule of differentiation.

Part 1

Let I=∫x2−a2 dxI = \displaystyle\int\sqrt{x^2-a^2}\,dx.

Integrate by parts with u=x2−a2u=\sqrt{x^2-a^2}, dv=dxdv=dx (so v=xv=x):

I=xx2−a2−∫x⋅xx2−a2 dx=xx2−a2−∫x2x2−a2 dxI = x\sqrt{x^2-a^2} - \int x\cdot\dfrac{x}{\sqrt{x^2-a^2}}\,dx = x\sqrt{x^2-a^2} - \int\dfrac{x^2}{\sqrt{x^2-a^2}}\,dx

Write x2=(x2−a2)+a2x^2 = (x^2-a^2)+a^2:

x2x2−a2=x2−a2+a2x2−a2\dfrac{x^2}{\sqrt{x^2-a^2}} = \sqrt{x^2-a^2} + \dfrac{a^2}{\sqrt{x^2-a^2}}

So

I=xx2−a2−∫x2−a2 dx−a2∫dxx2−a2I = x\sqrt{x^2-a^2} - \int\sqrt{x^2-a^2}\,dx - a^2\int\dfrac{dx}{\sqrt{x^2-a^2}}

I=xx2−a2−I−a2log⁡∣x+x2−a2∣+C1I = x\sqrt{x^2-a^2} - I - a^2\log\left|x+\sqrt{x^2-a^2}\right| + C_1

(using the standard result ∫dxx2−a2=log⁡∣x+x2−a2∣+C\int\dfrac{dx}{\sqrt{x^2-a^2}}=\log\left|x+\sqrt{x^2-a^2}\right|+C)

Bringing II to one side:

2I=xx2−a2−a2log⁡∣x+x2−a2∣+C12I = x\sqrt{x^2-a^2} - a^2\log\left|x+\sqrt{x^2-a^2}\right| + C_1

I=12[xx2−a2−a2log⁡∣x+x2−a2∣]+CI = \dfrac12\left[x\sqrt{x^2-a^2} - a^2\log\left|x+\sqrt{x^2-a^2}\right|\right] + C

Proved.


OR Part 2

Statement: If uu and vv are two functions of xx, then

∫u v dx=u∫v dx−∫[dudx∫v dx]dx\int u\,v\,dx = u\int v\,dx - \int\left[\dfrac{du}{dx}\int v\,dx\right]dx

i.e. "Integral of the product of two functions = (first function)×\times(integral of second) −- Integral of [(derivative of first)×\times(integral of second)]".

Proof: By the product rule of differentiation: …

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