Q.Integrate the following functions w.r.t. x:
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)∣
If stuck, differentiate a candidate "inside" function in your head. If its derivative (up to a constant) appears, that's your u.
The Definite Integral Case
Either change the limits (when x=a, u=g(a); when x=b, u=g(b); then integrate in u), or integrate in u, substitute back, and use the original limits. Changing limits is cleaner:
∫x=0x=12xcos(x2)dx=∫u=0u=1cos(u)du=sin(1)−sin(0)=sin(1)
Common Mistake to Avoid
Don't confuse du with Δu. du is a differential — the exact relationship du=g′(x)dx that holds inside the integral. Treat it algebraically: multiply, divide, and substitute freely.
U-substitution, taught in the CBSE Class 12 Integrals chapter as the method of substitution, is one of the very first integration techniques students learn after the standard formulas, and "integration by substitution class 12 examples" is a heavily searched revision topic. It remains equally essential for solving integral calculus problems in JEE Main and JEE Advanced.
Each integrand is an inner function times (a constant multiple of) its own derivative — a u-substitution.
(i) u=mx, du=mdx: ∫sinmxdx=−m1cosmx+C.
(ii) u=x2+1, du=2xdx: ∫2xsin(x2+1)dx=−cos(x2+1)+C.
(iii) u=tanx, so du=2xsec2xdx, giving xsec2xdx=2du:
∫xtan4xsec2xdx=∫u4⋅2du=52tan5x+C.
(iv) u=tan−1x, du=1+x2dx: ∫1+x2sin(tan−1x)dx=−cos(tan−1x)+C=−1+x21+C.
- −m1cosmx+C;
- −cos(x2+1)+C;
- 52tan5x+C;
- −cos(tan−1x)+C
All four are u-substitutions: (i) −mcosmx+C;
(ii) −cos(x2+1)+C;
(iii) 52tan5x+C;
(iv) −cos(tan−1x)+C=−1+x21+C.
The common idea
A u-substitution reverses the chain rule: if the integrand is f(g(x))g′(x), set u=g(x), du=g′(x)dx, and integrate f(u). In each part, find the inner function whose derivative is present (perhaps up to a constant).
(i) ∫sinmxdx
Let u=mx, so du=mdx, i.e. dx=mdu:
∫sinmxdx=m1∫sinudu=−m1cosu+C=−mcosmx+C.
Check: dxd(−mcosmx)=sinmx.
(ii) ∫2xsin(x2+1)dx
Here u=x2+1 has du=2xdx — exactly the factor present:
∫sinudu=−cosu+C=−cos(x2+1)+C.
(iii) ∫xtan4xsec2xdx
Take u=tanx. Then
du=sec2x⋅2x1dx⇒xsec2xdx=2du.
The integrand is tan4x⋅xsec2xdx=u4⋅2du, so
∫2u4du=52u5+C=52tan5x+C.
(iv) ∫1+x2sin(tan−1x)dx
Let u=tan−1x, so du=1+x2dx:
∫sinudu=−cosu+C=−cos(tan−1x)+C.
A right triangle with opposite x, adjacent 1, hypotenuse 1+x2 gives cos(tan−1x)=1+x21, so this is also −1+x21+C.
- −mcosmx+C;
- −cos(x2+1)+C;
- 52tan5x+C;
- −cos(tan−1x)+C=−1+x21+C
Method: Reverse Chain Rule (Spotting f(g(x))g′(x))
Use this for any integrand that is a composite function multiplied by (a constant times) the derivative of its inner part.
Steps
Step 1: Identify the inner function g(x).
Look for a function whose derivative is present in the integrand. Candidates: the argument of a trig function (mx, x2+1), or a nested expression such as tanx or tan−1x.
Step 2: Set u=g(x) and compute du.
Then du=g′(x)dx. Confirm the remaining factor in the integrand is du up to a constant. For u=mx, du=mdx, so a m1 is pulled out.
Step 3: Integrate in u and restore x.
The integral reduces to a standard form in u (e.g. ∫sinudu=−cosu, ∫u4du=5u5). Finish by back-substituting u=g(x) and adding C.
Common Mistakes
Mistake 1: Omitting the m1 in ∫sinmxdx.
Why it's wrong: du=mdx introduces a m1; forgetting it gives −cosmx instead of −mcosmx. Correct approach: always divide by the constant from du.
Mistake 2: Missing the "hidden" du in xsec2x.
Why it's wrong: with u=tanx, du=2xsec2xdx, so the whole factor is exactly 2du. Correct approach: differentiate the composite inner function fully before deciding the substitution.
Mistake 3: Not simplifying −cos(tan−1x).
Why it's wrong: leaving it unsimplified hides the neat closed form −1+x21. Correct approach: use cos(tan−1x)=1+x21.
Showing the 12 most recent of 32 on this concept.
- 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.
- That equals u^2/2 + C = (1/2)(sin^{-1}x)^2 + C.
Common Mistakes
- Forgetting the factor 1/2 from integrating u.
✓Final answerThe correct option is (A) — (1/2)(sin^{-1}x)^2 + C.
ANSWER: A
- 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.
∫1+x2sin(cot−1x)dx=∫sinu(−du)=−(−cosu)+C=cosu+C.
Back-substituting, =cos(cot−1x)+C.
✓Final answerThe correct option is (B).
- 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
cosx2sin2x1=2cosxsinxcosx1=2cos2xtanx1=2tanxsec2x.
Let u=tanx, du=sec2xdx:
21∫u−1/2du=21⋅2u1/2=u=tanx+C.
✓Final answerThe correct option is (B).
- 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
∫xx+1dx=∫u⋅2du=2⋅32u3/2+C=34(x+1)3/2+C.
✓Final answerThe correct option is (A).
- KEAM 2024Set eng-2024-06064 marksMCQQ.∫x4+3x2+1x2−1dx= (A) 31tan−1(3xx2+1)+C (B) tan−1(x2−1)+C (C) tan−1(x−x1)+C (D) 51tan−1(5xx2+1)+C (E) tan−1(x+x1)+C
›Reveal solutionSolution
The substitution u=x+x1 turns it into ∫u2+1du=tan−1(x+x1)+C.
Divide numerator and denominator by x2:
x4+3x2+1x2−1=x2+3+x211−x21.
Let u=x+x1, so du=(1−x21)dx and x2+x21=u2−2. The denominator becomes u2−2+3=u2+1.
Thus ∫u2+1du=tan−1u+C=tan−1(x+x1)+C.
✓Final answerThe correct option is (E).
- 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:
∫t+1dt=∫u+12udu=2∫(1−u+11)du=2(u−log(u+1))+C.
Thus the integral is 2(tanx−log(tanx+1))+C.
✓Final answerThe correct option is (A).
- 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.
∫tan12x+1tan5xsec2xdx=61∫u2+1du=61tan−1u+C=61tan−1(tan6x)+C.
✓Final answerThe correct option is (A).
- KEAM 2024Set eng-2024-06094 marksMCQQ.∫x2(x4+1)3/4dx= (A) −(x4+1)1/4+C (B) (x4+1)1/4+C (C) −(x4x4+1)1/4+C (D) (x4x4+1)+C (E) (x4x4+1)3/4+C
›Reveal solutionSolution
Factor x4 out of the radical; with u=1+x−4, du=−4x−5dx, the integral becomes −u1/4=−(x4x4+1)1/4+C.
Write (x4+1)3/4=x3(1+x−4)3/4, so
∫x2(x4+1)3/4dx=∫x5(1+x−4)3/4dx.
Let u=1+x−4, then du=−4x−5dx, i.e. x−5dx=−41du:
∫−41u−3/4du=−u1/4+C=−(1+x−4)1/4+C=−(x4x4+1)1/4+C.
✓Final answerThe correct option is (C).
- KEAM 2021Set eng-2021-P2-B14 marksMCQQ.∫cos2xcos(tanx)dx= (A) (tanx)sin(tanx)+C (B) sin(tanx)+C (C) sec(tanx)+C (D) (cosx)sin(tanx)+C (E) cos2(tanx)+C
›Reveal solutionSolution
∫cos2xcos(tanx)dx=sin(tanx)+C.
Concept and Intuition
The factor 1/cos2x=sec2x is exactly the derivative of tanx, so substituting u=tanx linearizes the integral.
Step-by-Step Solution
- Rewrite as ∫cos(tanx)sec2xdx.
- Let u=tanx, du=sec2xdx.
- Integral =∫cosudu=sinu+C=sin(tanx)+C.
Common Mistakes
- Failing to recognize 1/cos2x=sec2x as d(tanx).
✓Final answerThe correct option is (B) — sin(tanx)+C.
ANSWER: B
- KEAM 2024Set eng-2024-06064 marksMCQQ.∫4x2+74xcos4x2+7dx= (A) 21sin4x2+7+C (B) 27sin4x2+7+C (C) sin4x2+7+C (D) 41sin4x2+7+C (E) 47sin4x2+7+C
›Reveal solutionSolution
With u=4x2+7 the integrand is exactly cosudu, giving sin4x2+7+C.
Let u=4x2+7. Then dxdu=24x2+78x=4x2+74x, so du=4x2+74xdx.
The integral becomes ∫cosudu=sinu+C=sin4x2+7+C.
✓Final answerThe correct option is (C).
- 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
- Not recognising the 2tdt pattern for the log term.
- Missing the 1/2 factor in the arctan term.
✓Final answerThe correct option is (B) — 23tan−1(2tanx)+log∣tan2x+2∣+C.
ANSWER: B
- 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
- Miscounting powers so the sec2 factor is not isolated.
- Forgetting the 261 from integrating u25.
✓Final answerThe correct option is (D) — 26tan26(x)+C.
ANSWER: D
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