Q.Integrate the following function: 4x−x2x+2
🔒You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.
🔒 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)∣ …
Complete the square in the denominator and rewrite the numerator to match the derivative of the radicand.
4x−x2=4−(x−2)2,dxd(4x−x2)=4−2x.
Write x+2=−21(4−2x)+4 (check: −2+x+4=x+2). Then
∫4x−x2x+2dx=−21∫4x−x24−2xdx+4∫4−(x−2)2dx.
First integral: with u=4x−x2, du=(4−2x)dx, it is −21⋅2u=−4x−x2. …
Split x+2=−21(4−2x)+4; the first part is a u-substitution giving −4x−x2, the second a standard arcsine giving 4arcsin(2x−2). Result: −4x−x2+4arcsin(2x−2)+C.
Setting up
A square root of a quadratic invites completing the square:
4x−x2=−(x2−4x)=−((x−2)2−4)=4−(x−2)2.
So 4x−x2=4−(x−2)2, the classic a2−u2 shape with a=2.
The numerator split
The derivative of the radicand is dxd(4x−x2)=4−2x. We peel that off the numerator: solve x+2=A(4−2x)+B. Matching x: 1=−2A⇒A=−21; matching constants: 2=4A+B=−2+B⇒B=4. Hence
x+2=−21(4−2x)+4.
Step 1 — the derivative part
−21∫4x−x24−2xdx.
Let u=4x−x2, so du=(4−2x)dx:
−21∫udu=−21⋅2u=−4x−x2.
Step 2 — the arcsine part …
Method: quadraticlinear with a negative x2 term
Same split as any linear-over-root problem, but because the x2 coefficient is negative the constant piece becomes an inverse sine, and the derivative of the radicand carries a leading minus sign.
Steps
Step 1: Split the numerator. For Q(x)=c+bx−x2, note Q′(x)=b−2x; write the numerator as λ(b−2x)+μ.
Step 2: Derivative part. …
Common Mistakes
Mistake 1: Getting the sign of λ wrong.
Why it's wrong: the derivative of 4x−x2 is 4−2x, so x+2=−21(4−2x)+4; missing the minus flips the Q term's sign. Correct approach: differentiate the radicand exactly (with its minus) before matching.
Mistake 2: Using a log form for the constant part.
Why it's wrong: the negative x2 term gives a2−u2, hence sin−1. Correct approach: complete the square and check you have a2−u2, not u2±a2. …
Showing the 12 most recent of 16 on this concept.
- GUJCET 2022Set 081 markMCQQ.∫1−cos3xcosx−cos3xdx= ______ +C. (A) −23cos−1(cos3/2x) (B) −32cos−1(cos3/2x) (C) 23cos−1(cos3/2x) (D) 32cos−1(cos3/2x)
›Reveal solutionSolution
Simplify cosx−cos3x=cosxsin2x and recognise the derivative of cos−1(cos3/2x).
Concept. Numerator =cosx(1−cos2x)=cosxsin2x, so the integrand is
1−cos3xcosxsin2x=1−cos3xsinxcos1/2x. …
- GUJCET 2020Set 071 markMCQQ.∫cosxsinxcotxdx= ________ +C. (A) −2cotx (B) −2tanx (C) 2cotx (D) cotx1
›Reveal solutionSolution
Substitute t=cotx.
Concept: Rewrite sinxcosx1=cotxcsc2x and substitute.
Since cosx=cotxsinx, we have sinxcosx=sin2xcotx, so
sinxcosxcotx=cotxcotxcsc2x=(cotx)−1/2csc2x …
- GUJCET 2022Set 081 markMCQQ.∫(x+1)(x+3)(x+2)7dx= ______ +C. (A) 10(x+3)10+8(x+3)8 (B) 10(x+2)10+8(x+2)8 (C) 10(x+3)10−8(x+3)8 (D) 10(x+2)10−8(x+2)8
›Reveal solutionSolution
Centering at t=x+2 turns (x+1)(x+3) into t2−1.
Concept. Let t=x+2, so x+1=t−1, x+3=t+1, and (t−1)(t+1)=t2−1. …
- GUJCET 2022Set 081 markMCQQ.∫esinxsin2xdx= ______ +C. (A) esinx(sinx+1) (B) 2esinx(sinx−1) (C) 2esinx(sinx+1) (D) esinx(sinx−1)
›Reveal solutionSolution
sin2x=2sinxcosx turns the integral into 2∫ueudu=2eu(u−1).
Concept. ∫esinxsin2xdx=∫esinx2sinxcosxdx.
Let u=sinx, du=cosxdx: …
- GUJCET 2019Set 171 markMCQQ.If ∫sin13xcos3xdx=Asin14x+Bsin16x+C, then A+B= (A) 11217 (B) 11215 (C) 1101 (D) 1121
›Reveal solutionSolution
Substituting u=sinx gives ∫u13(1−u2)du=14sin14x−16sin16x, so A+B=1121.
Concept: cos3x=(1−sin2x)cosx. Let u=sinx, du=cosxdx:
∫u13(1−u2)du=14u14−16u16 …
- GUJCET 2023Set 091 markMCQQ.∫x2019⋅ex2020dx= ______ +C. (A) 20191ex2019 (B) 20201ex2019 (C) ex2020 (D) 20201ex2020
›Reveal solutionSolution
The x²⁰¹⁹ factor is (up to a constant) the derivative of the exponent x²⁰²⁰.
Concept. Substitution u=x2020, du=2020x2019dx.
Solution. …
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.∫x4−x2sec−1xdx= ____ +C.(a) −sec−1x(b) sec−1x(c) −21(sec−1x)2(d) 21(sec−1x)2
›Reveal solutionSolution
Recognize the integrand as f(x)f′(x) where f(x)=sec−1x.
x4−x2=∣x∣x2−1, and dxd(sec−1x)=∣x∣x2−11.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.∫(4x2+1)6x9dx = ____ +C.(a) 5x1(4+x21)−5(b) 10x1(x21+4)−5(c) 51(4+x21)−5(d) 101(x21+4)−5
›Reveal solutionSolution
Factor x12 out of the denominator and substitute t=4+x21.
Write (4x2+1)6x9=x12(4+x21)6x9=x−3(4+x21)−6.
Let t=4+x21, so dt=−x32dx, i.e. x−3dx=−2dt.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.∫cos26xsin24xdx = ____ +C.(a) 24tan24x(b) 26tan26x(c) 25tan25x(d) 27tan27x
›Reveal solutionSolution
Split off sec2x and write the rest in terms of tanx.
cos26xsin24x=tan24x⋅sec2x.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.∫3+4cos2xsinxdx = ____ +C.(a) log(3+4cos2x)(b) 231tan−1(23secx)(c) −231tan−1(3cosx)(d) 231tan−1(32cosx)
›Reveal solutionSolution
Substitute u=cosx to turn this into a standard ∫a2+b2u2du integral.
Let u=cosx, so du=−sinxdx, i.e. sinxdx=−du.
∫3+4cos2xsinxdx=−∫3+4u2du=−41∫u2+3/4du
=−41⋅3/21tan−1(3/2u)=−231tan−1(32u)+C
=−231tan−1(32cosx)+C.
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.∫cos2(x⋅ex)ex(1+x)dx = ____ +C.(a) −cot(x⋅ex)(b) tan(ex)(c) tan(x⋅ex)(d) cot(ex)
›Reveal solutionSolution
Recognize that ex(1+x) is the derivative of x⋅ex, so substitute t=xex.
dxd(xex)=ex+xex=ex(1+x).
…
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.∫ex+e−xdx = ____ +C.(a) tan−1(ex)(b) log(ex−e−x)(c) tan−1(e−x)(d) log(ex+e−x)
›Reveal solutionSolution
Multiply through by ex to turn the denominator into 1+e2x, a standard arctan form.
ex+e−x1=e2x+1ex.
…
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.