Q.Integrate the function cos3xelogsinx
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — U Substitution
U Substitution: The Reverse Chain Rule
The chain rule differentiates composite functions: the derivative of sin(x2) is cos(x2)⋅2x — differentiate the outer function, then multiply by the derivative of the inside. Integration asks the reverse: given cos(x2)⋅2x, find the original function. That's what u substitution does — it reverses the chain rule.
The Core Intuition
When an integral looks like "a function times the derivative of its inside," substitute the inside with u and the derivative of the inside with du. Consider:
∫2xcos(x2)dx
Here 2x is the derivative of x2, and x2 is the inside of cos(x2). Let u=x2, so du=2xdx:
∫cos(u)du=sin(u)+C=sin(x2)+C
Check: the derivative of sin(x2) is cos(x2)⋅2x.
The Precise Statement
∫f(g(x))⋅g′(x)dx=∫f(u)duwhere u=g(x),du=g′(x)dx
Valid provided g is differentiable and the resulting integral in u is simpler.
The Step-by-Step Method
- Identify a function g(x) whose derivative g′(x) also appears (possibly up to a constant factor).
- Set u=g(x), compute du=g′(x)dx.
- Rewrite the entire integral in u and du — every x and dx must be replaced.
- Integrate with respect to u.
- Substitute back u=g(x).
You cannot mix variables. If any x remains after substitution, you chose the wrong u (or must solve for x in terms of u — rare).
A Second Example (with a constant factor)
Evaluate ∫xx2+1dx. Let u=x2+1, so xdx=21du:
∫u⋅21du=21⋅32u3/2+C=31(x2+1)3/2+C
When Does It Work?
When the integrand is something times the derivative of something inside. Common patterns:
- x⋅f(x2) — derivative of x2 is 2x, so u=x2
- eg(x)⋅g′(x) — derivative of g(x) appears
- g(x)g′(x) — leads to log∣g(x)∣ …
Simplify the exponential first: elogsinx=sinx, so the integral is
∫cos3xsinxdx.
Let u=cosx, du=−sinxdx: …
Since elogsinx=sinx, the integral is ∫cos3xsinxdx; with u=cosx this gives −4cos4x+C.
1. Undo the exp-log
Exponential and natural log are inverses, so elogsinx=sinx (for sinx>0). The integrand collapses to
cos3xsinx.
2. Substitute
The derivative of cosx is −sinx, and a lone sinx is present, so a clean substitution works. Let u=cosx, so du=−sinxdx, i.e. sinxdx=−du:
∫cos3xsinxdx=∫u3(−du)=−∫u3du=−4u4+C. …
Method: Undo elog(⋅), then substitute
Use this when an exp-of-log wrapper hides a simple factor: cancel it first, then look for a derivative pair to substitute.
Steps
Step 1: Collapse the exponential-log.
elogg(x)=g(x) (valid where g(x)>0). This turns the integrand into an ordinary product of trig powers.
Step 2: Identify the substitution. …
Common Mistakes
Mistake 1: Not simplifying elogsinx=sinx.
Why it's wrong: leaving the exp-log wrapper hides that the integrand is just cos3xsinx. Correct approach: cancel the exponential and logarithm first.
Mistake 2: Dropping the minus sign from du=−sinxdx. …
Showing the 12 most recent of 51 on this concept.
- AP EAPCET 2026Set eng-2026-05-12-AN1 markMCQQ.If ∫esin3x(sin8x+2sin5x)cosxdx=31esin3xf(x)+c, then f(x)= (A) sin8x (B) sin6x (C) cos8x (D) cos6x
›Reveal solutionSolution
Substituting t=sinx and testing f(t)=t6 against the target derivative confirms f(x)=sin6x.
Concept and Intuition
When an integral has the form eg(x)⋅(stuff)⋅g′(x), substituting u=g(x) turns it into ∫eu⋅h(u)du for a polynomial h. Matching the answer's assumed shape 31euf and differentiating (product rule, since both eu and f depend on u) lets us solve for f by comparing polynomial coefficients — much safer than guessing an antiderivative by inspection alone.
Step-by-Step Solution
- Let t=sinx, so dt=cosxdx. The integral becomes ∫et3(t8+2t5)dt.
- We're told this equals 31et3f(t)+c. Differentiate the RHS w.r.t. t: dtd[31et3f(t)]=31[3t2et3f(t)+et3f′(t)]=et3[t2f(t)+31f′(t)].
- This must equal et3(t8+2t5), so t2f(t)+31f′(t)=t8+2t5. …
- AP EAPCET 2021Set eng-2021-10-05-FN1 markMCQQ.∫sin3xsinxdx= (A) 231log3−tanx3+tanx+c (B) 231log3+tanx3−tanx+c (C) 431log3−tanx3+tanx+c (D) 431log3+tanx3−tanx+c
›Reveal solutionSolution
Use the triple-angle identity to cancel sinx, rewrite in terms of cos2x, then apply the Weierstrass-type substitution t=tanx to reduce to a standard rational integral. The answer is (A).
Concept and Intuition
sin3x factors as sinx(3−4sin2x), so sinx cancels immediately with the numerator, turning a trigonometric-looking integral into a much simpler one in sin2x (hence in cos2x), which is a textbook target for the t=tanx substitution.
Step-by-Step Solution
- sin3x=3sinx−4sin3x=sinx(3−4sin2x).
- sin3xsinx=3−4sin2x1.
- Using sin2x=21−cos2x: 4sin2x=2−2cos2x, so 3−4sin2x=1+2cos2x.
- Integral becomes ∫1+2cos2xdx.
- Substitute t=tanx, dx=1+t2dt, cos2x=1+t21−t2: 1+2cos2x=1+t2(1+t2)+2(1−t2)=1+t23−t2.
- Integral =∫3−t21+t2⋅1+t2dt=∫3−t2dt. …
- AP EAPCET 2026Set eng-2026-05-12-FN1 markMCQQ.∫log2log3e2x−1e3x−3exdx= (A) log(32e) (B) log(34e) (C) 32e (D) 34e
›Reveal solutionSolution
A substitution t=ex turns this exponential integral into a rational one; the value works out to log(2e/3).
Concept and Intuition
Whenever an integral is built entirely from powers of ex, substituting t=ex converts it into an algebraic (rational function) integral, which is usually far easier to handle with partial fractions.
Step-by-Step Solution
- Let t=ex, so dt=exdx=tdx, i.e. dx=dt/t. When x=log2, t=2; when x=log3, t=3.
- Rewrite the integrand: e2x−1e3x−3ex=t2−1t3−3t. Multiplying by dx=dt/t gives t(t2−1)t3−3tdt=t2−1t2−3dt.
- So the integral becomes ∫23t2−1t2−3dt.
- Split: t2−1t2−3=t2−1(t2−1)−2=1−t2−12. …
- AP EAPCET 2026Set eng-2026-05-15-FN1 markMCQQ.∫10+sin2xcosx−sinxdx= (A) 21log(10+sin2x)+c (B) 31log(10+sin2x)+c (C) 31Tan−1(3sinx+cosx)+c (D) 31Tan−1(10+sin2x)+c
›Reveal solutionSolution
The numerator cosx−sinx is exactly d(sinx+cosx), and the denominator rewrites in terms of u=sinx+cosx via sin2x=u2−1. That collapses the integral to a standard ∫du/(a2+u2) arctangent form. Answer: 31Tan−1(3sinx+cosx)+c.
Concept and Intuition
Whenever an integrand contains both sinx−cosx (or cosx−sinx) and sin2x, it is worth trying u=sinx+cosx (or sinx−cosx) as the substitution, because u2=1±sin2x links the two.
Step-by-Step Solution
- Let u=sinx+cosx. Then du=(cosx−sinx)dx — this is exactly the numerator times dx.
- Also u2=sin2x+cos2x+2sinxcosx=1+sin2x, so sin2x=u2−1.
- Denominator: 10+sin2x=10+u2−1=9+u2.
- The integral becomes ∫9+u2du=31Tan−1(3u)+c. …
- AP EAPCET 2026Set eng-2026-05-15-FN1 markMCQQ.∫(1+sinx)4cos3xdx= (A) −5(1+sinx)5cos4x+c (B) 5(1+sinx)5cos4x+c (C) 4(1+sinx)4cos4x+c (D) −4(1+sinx)4cos4x+c
›Reveal solutionSolution
Factor cos3x using cos2x=(1−sinx)(1+sinx) and substitute t=sinx; the resulting antiderivative can equivalently be written in the cos4x/(1+sinx)4 form given in the options (they differ only by an added constant). Answer: −4(1+sinx)4cos4x+c.
Concept and Intuition
Integrals of cosoddx over powers of (1+sinx) are handled by peeling off one factor of cosx to pair with dx (making d(sinx)) and expressing the remaining even power of cosx in terms of sinx. Since the MCQ options are phrased in terms of cos4x rather than sinx directly, it is often faster (and safer against sign traps) to guess-and-check an antiderivative of that shape by differentiating a general form Acos4x(1+sinx)−n and matching powers/coefficients — this is exactly how the printed option is confirmed.
Step-by-Step Solution
- cos3x=cosx⋅cos2x=cosx(1−sin2x)=cosx(1−sinx)(1+sinx).
- Integrand =(1+sinx)4cosx(1−sinx)(1+sinx)=(1+sinx)3cosx(1−sinx).
- Let t=sinx, dt=cosxdx: I=∫(1+t)31−tdt. Writing 1−t=2−(1+t): I=∫((1+t)32−(1+t)21)dt=−(1+t)21+1+t1+c=(1+t)2t+c.
- So I=(1+sinx)2sinx+c is one valid closed form. …
- AP EAPCET 2021Set eng-2021-08-23-AN1 markMCQQ.∫2/e1/ex(logx)1/31dx= (A) 23{1+(log(2)−1)2/3} (B) 1 (C) 23{1+(log(2)+1)3/2} (D) 23{1−(log(2)−1)2/3}
›Reveal solutionSolution
Substitute u=logx to turn the integral into ∫u−1/3du, then evaluate between the transformed limits u=log2−1 and u=−1.
Concept and Intuition
An integrand of the form x⋅g(logx)1 always calls for the substitution u=logx, since du=dx/x removes the x and 1/x entirely, leaving a pure power of u.
Step-by-Step Solution
- Let u=logx, so du=dx/x.
- Limits: at x=2/e, u=log(2/e)=log2−1. At x=1/e, u=log(1/e)=−1.
- The integral becomes ∫log2−1−1u−1/3du=[2/3u2/3]log2−1−1=23[u2/3]log2−1−1.
- Using the real cube root, (−1)2/3=((−1)1/3)2=(−1)2=1.
- So the value is 23[1−(log2−1)2/3].
Common Mistakes …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.∫cos6x+sin6xsin2xcos2xdx= (A) 21Tan−1(tan2x)+c (B) 31Tan−1(tan2x)+c (C) 31Tan−1(tan3x)+c (D) Tan−1(tan3x)+c
›Reveal solutionSolution
Dividing through by cos6x to introduce tanx, then a double substitution (t=tanx, then u=t3), reduces the integral to a standard arctangent form, giving 31Tan−1(tan3x)+c.
Concept and Intuition
When an integrand has sin and cos appearing only in even powers that can be grouped, dividing everything by the highest power of cosx converts the whole expression into a rational function purely of tanx — a very common trick that opens the door to the substitution t=tanx. Here, since the denominator naturally forms 1+tan6x, a further substitution u=t3 turns it into the standard ∫1+u2du arctangent integral.
Step-by-Step Solution
- Divide numerator and denominator of cos6x+sin6xsin2xcos2x by cos6x: numerator becomes cos6xsin2xcos2x=cos4xsin2x=tan2xsec2x; denominator becomes 1+tan6x.
- The integral is now ∫1+tan6xtan2xsec2xdx.
- Substitute t=tanx, so dt=sec2xdx. The integral becomes ∫1+t6t2dt.
- Substitute u=t3, so du=3t2dt, i.e. t2dt=3du. The integral becomes 31∫1+u2du. …
- AP EAPCET 2025Set eng-2025-05-22-AN1 markMCQQ.If ∫sin2x+sin4xcos3xdx=c−cosecx−f(x), then f(2π)= (A) 1 (B) 0 (C) 2π (D) π
›Reveal solutionSolution
Substituting s=sinx and partial-fractioning gives −cosecx−2Tan−1(sinx), so f(x)=2Tan−1(sinx) and f(π/2)=π/2.
Concept and Intuition
Writing cos3xdx=cos2x⋅cosxdx=(1−sin2x)d(sinx) converts a trig integral into an algebraic one in s=sinx, which is then handled by ordinary partial fractions.
Step-by-Step Solution
- Let s=sinx, so ds=cosxdx, and cos3xdx=(1−s2)ds.
- The integral becomes ∫s2+s41−s2ds=∫s2(1+s2)1−s2ds.
- Partial fractions (in u=s2): u(1+u)1−u=u1−1+u2, so the integrand is s21−1+s22.
- Integrating: ∫(s21−1+s22)ds=−s1−2Tan−1s+C=−cosecx−2Tan−1(sinx)+C. …
- AP EAPCET 2021Set eng-2021-08-25-AN1 markMCQQ.∫cosxdx= (A) 2xsinx+2cosx+c (B) 2xsinx+2sinx+c (C) 2xsinx−2cosx+c (D) xcosx−2sinx+c
›Reveal solutionSolution
Substitute t=x to turn the integral into a standard integration-by-parts problem. Answer: 2xsinx+2cosx+c.
Concept and Intuition
Whenever you see x trapped inside a trig or exponential function, substituting t=x converts it into a polynomial-times-trig integral solvable by parts.
Step-by-Step Solution
- Let t=x⇒x=t2, dx=2tdt.
- ∫cosxdx=∫cost⋅2tdt=2∫tcostdt.
- Integrate by parts: ∫tcostdt=tsint−∫sintdt=tsint+cost. …
- AP EAPCET 2022Set eng-2022-07-06-AN1 markMCQQ.∫(sin2x+sin−3xcos5x)3cos4xdx= (A) 51(1+cot5x)−2+C (B) 101(1+cot2x)−5+C (C) 101(1+cot5x)−2+C (D) 51(1+cot5x)−5+C
›Reveal solutionSolution
Factoring sin2x out of the denominator turns it into sin2x(1+cot5x), and the substitution t=1+cot5x makes the whole integral a simple power-rule integration, giving 101(1+cot5x)−2+C.
Concept and Intuition
Integrals with mixed powers of sinx and cosx in odd/negative combinations often simplify beautifully once you factor out a common power to expose a (1+cotnx) or (1+tannx) structure — this is exactly the kind of expression whose derivative (via chain rule) reproduces cotn−1xcsc2x, matching what's left over in the integrand.
Step-by-Step Solution
- Denominator: sin2x+sin−3xcos5x. Factor out sin2x: =sin2x[1+sin5xcos5x]=sin2x(1+cot5x).
- So the full denominator cubed: [sin2x(1+cot5x)]3=sin6x(1+cot5x)3.
- Integrand: sin6x(1+cot5x)3cos4x=sin4xcos4x⋅sin2x1⋅(1+cot5x)−3=cot4xcsc2x(1+cot5x)−3.
- Substitute t=1+cot5x. Then dxdt=5cot4x⋅(−csc2x)=−5cot4xcsc2x, so cot4xcsc2xdx=−5dt. …
- AP EAPCET 2023Set eng-2023-05-18-FN1 markMCQQ.If ∫x(1−x3)2−1dx=32g(f(x))+c, then (A) f(x)=x, g(x)=sin−1x (B) f(x)=x3/2, g(x)=sin−1x (C) f(x)=x3/2, g(x)=cos−1x (D) f(x)=x, g(x)=cos−1x
›Reveal solutionSolution
A substitution u=x3/2 turns the integral into the standard ∫du/1−u2 form, giving f(x)=x3/2 and g=sin−1.
Concept and Intuition
The presence of xdx alongside x3=(x3/2)2 inside a square root strongly signals the substitution u=x3/2 (its derivative is proportional to x, exactly what's needed to absorb the leftover xdx). Once substituted, the integral collapses to the standard arcsine form.
Step-by-Step Solution
- Let u=x3/2. Then du=23x1/2dx=23xdx, so xdx=32du.
- Also, u2=x3, so 1−x3=1−u2.
- Substitute into the integral: ∫x(1−x3)−1/2dx=∫32⋅1−u2du=32∫1−u2du.
- This is the standard form: ∫1−u2du=sin−1u+c.
- So the integral =32sin−1(u)+c=32sin−1(x3/2)+c. …
- AP EAPCET 2024Set eng-2024-05-20-AN1 markMCQQ.∫sin3xcos(x−α)dx= (A) cosα1cotx+tanα+c (B) cosα1cotx−tanα+c (C) sinα−1cotx+tanα+c (D) cosα−2cotx+tanα+c
›Reveal solutionSolution
Expanding cos(x−α) and substituting u=cotx reduces this to a simple square-root integral; final answer is (D).
Concept and Intuition
The key move is expanding cos(x−α) and factoring out sin4x from inside the square root so that a substitution u=cotx (whose differential is −csc2xdx, conveniently matching what's left outside) linearizes the whole thing.
Step-by-Step Solution
- cos(x−α)=cosxcosα+sinxsinα.
- sin3xcos(x−α)=sin3xcosxcosα+sin4xsinα=sin4x(sinxcosxcosα+sinα)=sin4x(cotxcosα+sinα).
- So sin3xcos(x−α)=sin2xcotxcosα+sinα (taking sin2x>0 outside the root).
- The integral is ∫sin2xcosαcotx+sinαdx=∫cosαcotx+sinαcsc2xdx.
- Let u=cotx, du=−csc2xdx. Integral becomes −∫cosαu+sinαdu=−cosα2cosαu+sinα+c. …
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