Q.β« π
π ππ(π+ππ) π π equals
(A) β 1 2π₯2 β1 + π₯4 + π
(B) 1 2π₯ β1 + π₯4 + π
(C) β 1 4π₯ β1 + π₯4 + π
(D) 1 4π₯2 β1 + π₯4 + π
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π Start your 14-day free trial to unlock the full solution βConcept understanding β U Substitution
U Substitution: The Reverse Chain Rule
The chain rule differentiates composite functions: the derivative of sin(x2) is cos(x2)β 2x β differentiate the outer function, then multiply by the derivative of the inside. Integration asks the reverse: given cos(x2)β 2x, find the original function. That's what u substitution does β 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:
β«2xcos(x2)dx
Here 2x is the derivative of x2, and x2 is the inside of cos(x2). Let u=x2, so du=2xdx:
β«cos(u)du=sin(u)+C=sin(x2)+C
Check: the derivative of sin(x2) is cos(x2)β 2x.
The Precise Statement
β«f(g(x))β gβ²(x)dx=β«f(u)duwhereΒ u=g(x),du=gβ²(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 gβ²(x) also appears (possibly up to a constant factor).
- Set u=g(x), compute du=gβ²(x)dx.
- Rewrite the entire integral in u and du β 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 β rare).
A Second Example (with a constant factor)
Evaluate β«xx2+1βdx. Let u=x2+1, so xdx=21βdu:
β«uββ 21βdu=21ββ 32βu3/2+C=31β(x2+1)3/2+C
When Does It Work?
When the integrand is something times the derivative of something inside. Common patterns:
- xβ f(x2) β derivative of x2 is 2x, so u=x2
- eg(x)β gβ²(x) β derivative of g(x) appears
- g(x)gβ²(x)β β leads to logβ£g(x)β£ β¦
Key idea: factor x4 out of the root, then the leftover is a perfect differential.
Since 1+x4β=x21+xβ4β, the integrand becomes
x31+x4β1β=x3β x21+xβ4β1β=1+xβ4βxβ5β.
Let u=1+xβ4, so du=β4xβ5dx, i.e. xβ5dx=β41βdu:
β«1+xβ4βxβ5dxβ=β41ββ«uβ1/2du=β41ββ 2uβ=β21βuβ. β¦
Pull x4 out of the square root and substitute u=1+xβ4; the integral equals β2x21+x4ββ+c, which is option (A).
We want
β«x31+x4βdxβ.
Why factor x4 out? The derivative of x4 is 4x3, so a bare u=x4 substitution wants an x3 in the numerator β but here x3 sits in the denominator. Pulling x4 out of the root converts the problem into one where the exact needed differential does appear.
1. Rewrite the integrand
1+x4β=x4(1+x41β)β=x21+xβ4β(x>0).
So
x31+x4β1β=x3β x21+xβ4β1β=1+xβ4βxβ5β.
2. Substitute
Let u=1+xβ4. Then du=β4xβ5dx, so xβ5dx=β41βdu. Notice the integrand contains exactly xβ5dx times uβ1β:
β«1+xβ4βxβ5dxβ=β«uββ41βduβ=β41ββ«uβ1/2du. β¦
Method: Substitution when a high power of x blocks the obvious u
Use this for integrands like xm1+xnβ1β where a direct substitution u=1+xn fails because the needed xnβ1 sits in the denominator, not the numerator.
Steps
Step 1: Factor the highest power of x out of the root.
1+xnβ=xn(1+xβn)β=xn/21+xβnβ(x>0).
This deliberately introduces a negative power of x, which is the differential you actually need.
Step 2: Collect all powers of x into one factor.
Rewrite the whole integrand so it reads (power of x) Γ1+xβnβ1β. The power of x should now match the derivative of xβn.
Step 3: Substitute u=1+xβn. β¦
Common Mistakes
Mistake 1: Trying u=1+x4 directly.
Why it's wrong: then du=4x3dx needs an x3 in the numerator, but here x3 is in the denominator β the substitution leaves stray x's. Correct approach: factor x4 out of the root first to manufacture the xβ5dx that u=1+xβ4 needs.
Mistake 2: Mishandling x4β=x2 signs.
Why it's wrong: x4β=x2 is fine, but pulling out x-powers carelessly (e.g. x4β=x) corrupts the algebra. Correct approach: track exponents precisely β x4β=x2, and 1+xβ4β=1+x4β/x2. β¦
Showing the 12 most recent of 16 on this concept.
- GUJCET 2022Set 081 markMCQQ.β«1βcos3xcosxβcos3xββdx= ______ +C. (A) β23βcosβ1(cos3/2x) (B) β32βcosβ1(cos3/2x) (C) 23βcosβ1(cos3/2x) (D) 32βcosβ1(cos3/2x)
βΊReveal solutionSolution
Simplify cosxβcos3x=cosxsin2x and recognise the derivative of cosβ1(cos3/2x).
Concept. Numerator =cosx(1βcos2x)=cosxsin2x, so the integrand is
1βcos3xcosxsin2xββ=1βcos3xβsinxcos1/2xβ. β¦
- GUJCET 2020Set 071 markMCQQ.β«cosxsinxcotxββdx= ________ +C. (A) β2cotxβ (B) β2tanxβ (C) 2cotxβ (D) cotxβ1β
βΊReveal solutionSolution
Substitute t=cotx.
Concept: Rewrite sinxcosx1β=cotxcsc2xβ and substitute.
Since cosx=cotxsinx, we have sinxcosx=sin2xcotx, so
sinxcosxcotxββ=cotxcotxβcsc2xβ=(cotx)β1/2csc2x β¦
- GUJCET 2019Set 171 markMCQQ.If β«sin13xcos3xdx=Asin14x+Bsin16x+C, then A+B=β (A) 11217β (B) 11215β (C) 1101β (D) 1121β
βΊReveal solutionSolution
Substituting u=sinx gives β«u13(1βu2)du=14sin14xββ16sin16xβ, so A+B=1121β.
Concept: cos3x=(1βsin2x)cosx. Let u=sinx, du=cosxdx:
β«u13(1βu2)du=14u14ββ16u16β β¦
- GUJCET 2022Set 081 markMCQQ.β«(x+1)(x+3)(x+2)7dx= ______ +C. (A) 10(x+3)10β+8(x+3)8β (B) 10(x+2)10β+8(x+2)8β (C) 10(x+3)10ββ8(x+3)8β (D) 10(x+2)10ββ8(x+2)8β
βΊReveal solutionSolution
Centering at t=x+2 turns (x+1)(x+3) into t2β1.
Concept. Let t=x+2, so x+1=tβ1, x+3=t+1, and (tβ1)(t+1)=t2β1. β¦
- GUJCET 2023Set 091 markMCQQ.β«x2019β ex2020dx= ______ +C. (A) 20191βex2019 (B) 20201βex2019 (C) ex2020 (D) 20201βex2020
βΊReveal solutionSolution
The xΒ²β°ΒΉβΉ factor is (up to a constant) the derivative of the exponent xΒ²β°Β²β°.
Concept. Substitution u=x2020, du=2020x2019dx.
Solution. β¦
- GUJCET 2022Set 081 markMCQQ.β«esinxsin2xdx= ______ +C. (A) esinx(sinx+1) (B) 2esinx(sinxβ1) (C) 2esinx(sinx+1) (D) esinx(sinxβ1)
βΊReveal solutionSolution
sin2x=2sinxcosx turns the integral into 2β«ueudu=2eu(uβ1).
Concept. β«esinxsin2xdx=β«esinx2sinxcosxdx.
Let u=sinx, du=cosxdx: β¦
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.β«x4βx2βsecβ1xβdx= ____ +C.(a) βsecβ1x(b) secβ1x(c) β21β(secβ1x)2(d) 21β(secβ1x)2
βΊReveal solutionSolution
Recognize the integrand as f(x)fβ²(x) where f(x)=secβ1x.
x4βx2β=β£xβ£x2β1β, and dxdβ(secβ1x)=β£xβ£x2β1β1β.
β¦
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.β«(4x2+1)6x9βdx = ____ +C.(a) 5x1β(4+x21β)β5(b) 10x1β(x21β+4)β5(c) 51β(4+x21β)β5(d) 101β(x21β+4)β5
βΊReveal solutionSolution
Factor x12 out of the denominator and substitute t=4+x21β.
Write (4x2+1)6x9β=x12(4+x21β)6x9β=xβ3(4+x21β)β6.
Let t=4+x21β, so dt=βx32βdx, i.e. xβ3dx=β2dtβ.
β¦
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.β«cos26xsin24xβdx = ____ +C.(a) 24tan24xβ(b) 26tan26xβ(c) 25tan25xβ(d) 27tan27xβ
βΊReveal solutionSolution
Split off sec2x and write the rest in terms of tanx.
cos26xsin24xβ=tan24xβ sec2x.
β¦
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.β«3+4cos2xsinxβdx = ____ +C.(a) log(3+4cos2x)(b) 23β1βtanβ1(23βsecxβ)(c) β23β1βtanβ1(3βcosxβ)(d) 23β1βtanβ1(3β2cosxβ)
βΊReveal solutionSolution
Substitute u=cosx to turn this into a standard β«a2+b2u2duβ integral.
Let u=cosx, so du=βsinxdx, i.e. sinxdx=βdu.
β«3+4cos2xsinxβdx=ββ«3+4u2duβ=β41ββ«u2+3/4duβ
=β41ββ 3β/21βtanβ1(3β/2uβ)=β23β1βtanβ1(3β2uβ)+C
=β23β1βtanβ1(3β2cosxβ)+C.
β¦
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.β«cos2(xβ ex)ex(1+x)βdx = ____ +C.(a) βcot(xβ ex)(b) tan(ex)(c) tan(xβ ex)(d) cot(ex)
βΊReveal solutionSolution
Recognize that ex(1+x) is the derivative of xβ ex, so substitute t=xex.
dxdβ(xex)=ex+xex=ex(1+x).
β¦
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.β«ex+eβxdxβ = ____ +C.(a) tanβ1(ex)(b) log(exβeβx)(c) tanβ1(eβx)(d) log(ex+eβx)
βΊReveal solutionSolution
Multiply through by ex to turn the denominator into 1+e2x, a standard arctan form.
ex+eβx1β=e2x+1exβ.
β¦
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