Q.Find ∫[cotx+tanx]dx
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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)∣ …
Concept: method of substitution (combine the surds over a common denominator, then substitute t=sinx−cosx).
Write both surds over sinxcosx:
cotx+tanx=sinxcosx+cosxsinx=sinxcosxcosx+sinx.
Let t=sinx−cosx, so dt=(cosx+sinx)dx (exactly the numerator). Also
t2=(sinx−cosx)2=1−2sinxcosx ⇒ sinxcosx=21−t2.
Hence sinxcosx=21−t2, and …
Combine the two surds into sinxcosxsinx+cosx, then substitute t=sinx−cosx (so dt matches the numerator) to reach the standard integral ∫1−t2dt. Final answer: 2sin−1(sinx−cosx)+C.
Step 1 — Combine the surds
Write each term with sines and cosines and put them over a common denominator:
cotx+tanx=sinxcosx+cosxsinx=sinxcosxcosx+sinxcosxsinx=sinxcosxsinx+cosx.
Step 2 — Pick the substitution
The numerator sinx+cosx is the derivative of sinx−cosx. So let
t=sinx−cosx⇒dt=(cosx+sinx)dx.
Thus (sinx+cosx)dx=dt — the numerator is handled exactly.
Step 3 — Express the denominator through t
Squaring,
t2=(sinx−cosx)2=sin2x+cos2x−2sinxcosx=1−2sinxcosx,
so
sinxcosx=21−t2⇒sinxcosx=21−t2=21−t2.
Step 4 — Substitute into the integral
∫sinxcosxsinx+cosxdx=∫21−t2dt=2∫1−t2dt. …
Method: Combine tanx+cotx, then a t±t1 substitution
Use this for integrals of tanx±cotx: rewrite over a common denominator, substitute to a single variable, and force a standard ∫u2+a2du (or ∫a2−u2du) form.
Steps
Step 1: Merge the two roots.
Since cotx=tanx1,
cotx+tanx=tanx1+tanxor, in sin/cos,sinxcosxsinx+cosx.
Either single fraction is now substitutable.
Step 2: Substitute so the numerator becomes du.
Path A: let t=tanx, then dx=1+t42tdt and the integral becomes 2∫1+t41+t2dt. Path B: let t=sinx−cosx, whose derivative (cosx+sinx) is exactly the numerator.
Step 3: Reduce to a standard form. …
Common Mistakes
Mistake 1: Integrating tanx and cotx separately.
Why it's wrong: each root alone leads to a hard integral, whereas their sum combines into a single clean fraction. Correct approach: put them over a common denominator first (sinxcosxsinx+cosx) so a single substitution works.
Mistake 2: Choosing the substitution whose derivative does not match the numerator. …
Showing the 12 most recent of 32 on this concept.
- KEAM 2026Set eng-2026-04184 marksMCQQ.∫sinx+cosxsecxsecxdx= (A) 2(tanx−loge(tanx+1))+C (B) 2(tanx+loge(tanx+1))+C (C) 2tanx−loge(tanx+1)+C (D) 2tanx+loge(tanx+1)+C (E) tanx−loge(tanx+1)+C
›Reveal solutionSolution
Factor sinx+cosx=cosx(tanx+1) to reduce the integrand to tanx+1sec2x; substituting u=tanx gives option (A).
Write the denominator as sinx+cosx=cosx(tanx+1). The numerator is secxsecx=cos−3/2x, so
cosx(tanx+1)cos−3/2x=tanx+1cos−2x=tanx+1sec2x.
Let t=tanx, dt=sec2xdx, then u=t, t=u2, dt=2udu: …
- KEAM 2025Set eng-2025-04294 marksMCQQ.∫cosx2sin2xdx= (A) 21tanx+C (B) tanx+C (C) 2tanx+C (D) 4tanx+C (E) 3tanx+C
›Reveal solutionSolution
Writing 2sin2x=4sinxcosx reduces the integrand to 21sec2x(tanx)−1/2; with u=tanx this integrates to tanx+C.
Use sin2x=2sinxcosx, so 2sin2x=4sinxcosx and 2sin2x=2sinxcosx. Then …
- KEAM 2025Set eng-2025-04254 marksMCQQ.∫2x+5sec2(2x+5)dx= (A) 2tan(2x+5)+C (B) 21tan(2x+5)+C (C) tan(2x+5)+C (D) tan(2x+5)+C (E) 2tan(2x+5)+C
›Reveal solutionSolution
Substitute u=2x+5; the differential cancels the 2x+51 factor.
Let u=2x+5. Then
du=22x+51⋅2dx=2x+5dx.
So the integral becomes …
- KEAM 2026Set eng-2026-04174 marksMCQQ.∫xx+1dx= (A) 34(x+1)23+C (B) 32(x+1)23+C (C) 34(x+1)43+C (D) 31(x+1)23+C (E) 43(x+1)23+C
›Reveal solutionSolution
Substitute u=x+1.
With u=x+1, du=2x1dx, i.e. xdx=2du. Then …
- KEAM 2022Set eng-2022-P2-B14 marksMCQQ.∫cos27xsin25xdx is equal to (A) 26sin26(x)+C (B) 26cos26(x)+C (C) tan26(x)+C (D) 26tan26(x)+C (E) 26tan26(x)+C
›Reveal solutionSolution
The integral equals 26tan26x+C.
Concept and Intuition
Split off a sec2x factor so the rest becomes a power of tanx, then substitute u=tanx.
Step-by-Step Solution
- cos27xsin25x=tan25x⋅cos2x1=tan25xsec2x.
- Let u=tanx⇒du=sec2xdx.
- ∫tan25xsec2xdx=∫u25du=26u26+C.
- =26tan26x+C.
Common Mistakes …
- KEAM 2026Set eng-2026-04194 marksMCQQ.∫1+x2sin(cot−1x)dx is equal to (A) −cos(cot−1x)+C (B) cos(cot−1x)+C (C) 1+x2cos(cot−1x)+C (D) 2cos(cot−1x)+C (E) 1+x2−cos(cot−1x)+C
›Reveal solutionSolution
Substitute u=cot−1x so du=−1+x2dx.
Let u=cot−1x⇒du=−1+x2dx, i.e. 1+x2dx=−du. …
- KEAM 2024Set eng-2024-06054 marksMCQQ.∫tan12x+1tan5xsec2xdx is equal to (A) 61tan−1[tan6x]+C (B) 21tan−1[tan6x]+C (C) 41tan−1[tan4x]+C (D) 31tan−1[tan3x]+C (E) 71tan−1[tan7x]+C
›Reveal solutionSolution
Substitute u=tan6x; the integral reduces to 61∫u2+1du=61tan−1(tan6x)+C.
Let u=tan6x. Then du=6tan5xsec2xdx, so tan5xsec2xdx=6du.
Also tan12x=(tan6x)2=u2. …
- KEAM 2025Set eng-2025-04284 marksMCQQ.∫cos2/3xsin4/3xdx is (A) 3tan3x+C (B) 3tan1/3x+C (C) −3tan1/3x+C (D) −3tan−1/3x+C (E) 3tan−1/3x+C
›Reveal solutionSolution
Rewrite as sec2xtan−4/3xdx; sub t=tanx to get ∫t−4/3dt=−3t−1/3=−3tan−1/3x+C.
The integrand cos2/3xsin4/3x1 has denominator powers summing to 2, so factor out cos2x. Multiplying numerator and denominator by cos4/3x (equivalently dividing top and bottom by cos2x): …
- KEAM 2025Set eng-2025-04264 marksMCQQ.∫0π/21+sinx1dx= (A) 2 (B) 21 (C) 41 (D) 1 (E) 0
›Reveal solutionSolution
Rationalise the denominator, integrate sec2x−secxtanx.
Multiply numerator and denominator by 1−sinx:
1+sinx1=1−sin2x1−sinx=cos2x1−sinx=sec2x−secxtanx. …
- KEAM 2023Set eng-2023-P2-B24 marksMCQQ.∫sin2x+2cos2x2tanx+3dx= (A) 23sin−1(2sinx)+lnsin2x+2+C (B) 23tan−1(2tanx)+lntan2x+2+C (C) 21tan−1(2tanx)−lntan2x+2+C (D) 23cos−1(2cosx)+lnsin2x+2+C (E) 21cos−1(2cosx)−lncos2x+2+C
›Reveal solutionSolution
The integral is 23tan−1(2tanx)+log∣tan2x+2∣+C.
Concept and Intuition
Divide numerator and denominator by cos2x to convert everything into tanx, then substitute t=tanx.
Step-by-Step Solution
- Dividing by cos2x: integrand =tan2x+2(2tanx+3)sec2x.
- Let t=tanx,dt=sec2xdx: ∫t2+22t+3dt.
- Split: ∫t2+22tdt=log(t2+2) and ∫t2+23dt=23tan−12t.
Common Mistakes …
- KEAM 2025Set eng-2025-04234 marksMCQQ.∫1−x2sin−1xdx= (A) 21(sin−1x)2+C (B) −(sin−1x)1−x2+C (C) (sin−1x)1−x2+x+C (D) (sin−1x)1−x2−x+C (E) (sin−1x)2+C
›Reveal solutionSolution
Substituting u=sin^{-1}x turns the integral into u du = (1/2)(sin^{-1}x)^2 + C.
Concept and Intuition
The factor 1/sqrt(1-x^2) is exactly the derivative of sin^{-1}x, so the substitution u = sin^{-1}x collapses the integral.
Step-by-Step Solution
- Let u = sin^{-1}x, then du = dx/sqrt(1-x^2).
- The integral becomes integral of u du. …
- KEAM 2025Set eng-2025-04264 marksMCQQ.∫(secx+tanx)9secxdx= (A) 91(secx+tanx)9+C (B) 9−1(secx+tanx)9+C (C) 9−1(secx+tanx)−9+C (D) 91(secx+tanx)−9+C (E) (secx+tanx)−9+C
›Reveal solutionSolution
Substitute u=secx+tanx; then secxdx=du/u.
Let u=secx+tanx. Then du=(secxtanx+sec2x)dx=secx(tanx+secx)dx=secxudx, so secxdx=udu. …
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