The value of the integral ∫0π/23cos⁡θcos⁡θ+sin⁡θ5dθ equals

JEE Advanced · Mathematics · 27. Definite Integration

The value of the integral $\int_{0}^{π / 2} \frac{3 \sqrt{\cos θ}}{{(\sqrt{\cos θ} + \sqrt{\sin θ})}^{5}} d θ$ equals

A 0.3
B 0.4
C 0.5
D 0.9

Verified Solution

Answer & Solution

Correct Answer

(C) 0.5

Step-by-step Solution

Detailed explanation

I = \int_{0}^{\frac{π}{2}} \frac{3 \sqrt{\cos θ}}{{(\sqrt{\sin θ} + \sqrt{\cos θ})}^{5}} . d θ

…(i)

I = \int_{0}^{\frac{π}{2}} \frac{3 \sqrt{\sin θ}}{{(\sqrt{\cos θ} + \sqrt{\sin θ})}^{5}} . d θ

…(ii)

using \int_{a}^{b} f (x) . d x = \int_{a}^{b} f (a + b - x) . d x

(i)

+

(ii)

\Rightarrow 2 I = \int_{0}^{\frac{π}{2}} \frac{3 (\sqrt{\sin θ} + \sqrt{\cos θ})}{{(\sqrt{\sin θ} + \sqrt{\cos θ})}^{5}} . d θ

\Rightarrow 2 I = 3 \int_{0}^{\frac{π}{2}} \frac{d θ}{{(\sqrt{\sin θ} + \sqrt{\cos θ})}^{4}}

\Rightarrow \frac{2 I}{3} = \int_{0}^{\frac{π}{2}} \frac{\sec^{2} θ . d θ}{{(1 + \sqrt{\tan θ})}^{4}}

Put

\tan θ = t^{2}

\sec^{2} θ . d θ = 2 t . d t

when

\begin{matrix} θ \to 0, & t \to 0 \end{matrix}

\begin{matrix} θ \to \frac{π}{2}, & t \to \infty \end{matrix}

\Rightarrow \frac{2 I}{3} = \int_{0}^{\infty} \frac{2 t d t}{{(1 + t)}^{4}}

\Rightarrow \frac{I}{3} = \int_{0}^{\infty} \frac{(t + 1) - 1}{{(1 + t)}^{4}} . d t

\frac{I}{3} = \int_{0}^{\infty} \frac{1}{{(1 + t)}^{3}} - \frac{1}{{(1 + t)}^{4}} . d t

\frac{I}{3} = {[- \frac{1}{2 {(1 + t)}^{2}} + \frac{1}{3 {(1 + t)}^{3}}]}_{0}^{\infty}

\frac{I}{3} = [0 + 0 - (- \frac{1}{2} + \frac{1}{3})]

\frac{I}{3} = \frac{1}{6} \Rightarrow I = \frac{1}{2}

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

Same subject

Match the following.

	List – I		List – II
(A)	The number of polynomials f(x) with non – negative integer coefficients of degree $\leq 2,$ satisfying f(0) =0 and $\int_{0}^{1} f (x) d x = 1, i s$	(P)	8
(B)	The number of point in the interval $[- \sqrt{13}, \sqrt{13}]$ at which $f (x) = \sin (x^{2}) + \cos (x^{2})$ attains its maximum value, is	(Q)	2
(C)	$\int_{- 2}^{2} \frac{3 x^{2}}{(1 + e^{x})} d x$ equals	(R)	4
(D)	$\frac{(\int_{- \frac{1}{2}}^{\frac{1}{2}} \cos 2 x . \log (\frac{1 + x}{1 - x}) d x)}{(\int_{0}^{\frac{1}{2}} \cos 2 x . \log (\frac{1 + x}{1 - x}) d x)}$	(S)	0

JEE Advanced 2014 Hard

	List- I		List- II
$(I)$	$2 h + k$	$(P)$	$6$
$(I I)$	$\frac{L e n g t h o f Z W}{L e n g t h o f X Y}$	$(Q)$	$\sqrt{6}$
$(I I I)$	$\frac{A r e a o f t r i a n g l e M Z N}{A r e a o f t r i a n g l e Z M W}$	$(R)$	$\frac{5}{4}$
$(I V)$	$α$	$(S)$	$\frac{21}{5}$
		$(T)$	$2 \sqrt{6}$
		$(U)$	$\frac{10}{3}$

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