Q.Integrate the following function: (x−5)(x−4)6x+7
🔒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)∣ …
Concept: U Substitution – we rewrite the numerator as a derivative of the quadratic inside the square root, then integrate using ∫udu=2u.
First, expand the denominator: (x−5)(x−4)=x2−9x+20. Its derivative is 2x−9.
We want to express 6x+7 as A(2x−9)+B.
Comparing: 6x+7=2Ax−9A+B gives 2A=6⇒A=3, and −9(3)+B=7⇒B=34.
So the integral becomes:
∫x2−9x+203(2x−9)+34dx=3∫x2−9x+202x−9dx+34∫x2−9x+20dx
For the first term, let u=x2−9x+20, du=(2x−9)dx:
3∫udu=3⋅2u=6x2−9x+20
For the second term, complete the square: x2−9x+20=(x−29)2−41. …
Split 6x+7=3(2x−9)+34 so the numerator carries the derivative of the quadratic; the integral is 6x2−9x+20+34logx−29+x2−9x+20+C.
Expand the radicand. (x−5)(x−4)=x2−9x+20, whose derivative is 2x−9.
Split the numerator. Write 6x+7=A(2x−9)+B. Matching coefficients gives A=3 and −9(3)+B=7⇒B=34, so 6x+7=3(2x−9)+34. Hence
∫x2−9x+206x+7dx=3∫x2−9x+202x−9dx+34∫x2−9x+20dx.
First integral. With u=x2−9x+20, du=(2x−9)dx:
3∫udu=6u=6x2−9x+20. …
Method: quadraticlinear split — square-root part + log part
Break the linear numerator into a multiple of the radicand's derivative plus a constant; the derivative part integrates to 2Q and the constant part to a standard root-form integral.
Steps
Step 1: Split. For Q(x)=ax2+bx+c, write px+q=λ(2ax+b)+μ and solve for λ,μ.
Step 2: Derivative part.
λ∫Q(x)2ax+bdx=2λQ(x).
Step 3: Constant part — complete the square. …
Common Mistakes
Mistake 1: Matching the split against the wrong derivative.
Why it's wrong: the derivative of (x−5)(x−4)=x2−9x+20 is 2x−9, so 6x+7=3(2x−9)+34; using 2x−9 wrong drops or mis-scales the 2Q term. Correct approach: expand the product, differentiate, then split.
Mistake 2: Forgetting the factor 2 in the square-root part. …
Showing the 12 most recent of 32 on this concept.
- 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. …
- 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. …
- 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 2022Set eng-2022-P2-B14 marksMCQQ.∫x311−x21dx= (A) 6−1(1−x21)23+C (B) 31(1−x21)23+C (C) 3−1(1−x21)23+C (D) 34(1−x21)23+C (E) 3−4(1−x21)23+C
›Reveal solutionSolution
The integral equals 31(1−x21)3/2+C.
Concept and Intuition
The derivative of 1−x21 is x32, which matches the x31 factor outside the root, so a substitution linearizes the integral.
Step-by-Step Solution
- Let u=1−x21.
- du=x32dx⇒x3dx=2du.
- ∫x311−x21dx=∫u2du=21⋅3/2u3/2.
- =21⋅32u3/2=31u3/2=31(1−x21)3/2+C. …
- KEAM 2025Set eng-2025-04234 marksMCQQ.∫x7(x8+1)−3/4dx= (A) 21(1+x81)1/4+C (B) 4(1+x81)1/4+C (C) (x8+1)1/4+C (D) 4(x8+1)1/4+C (E) 21(x8+1)1/4+C
›Reveal solutionSolution
Substituting u=x^8+1 gives (1/2)(x^8+1)^{1/4} + C.
Concept and Intuition
The presence of x^7 alongside x^8 signals the substitution u = x^8 + 1, whose differential absorbs x^7 dx.
Step-by-Step Solution
- Let u = x^8 + 1, then du = 8 x^7 dx, so x^7 dx = du/8.
- Integral = (1/8) integral of u^{-3/4} du.
- = (1/8) * u^{1/4}/(1/4) = (1/8)4u^{1/4} = (1/2)u^{1/4}. …
- 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 2026Set eng-2026-04194 marksMCQQ.∫(13+36sin2tsint+cost)dt is equal to (A) 841log7−6(sint−cost)7+6(sint−cost)+C (B) 811log7−6(sint−cost)7+6(sint−cost)+C (C) 841log7+6(sint−cost)7−6(sint−cost)+C (D) 481log7−6(sint−cost)7+6(sint−cost)+C (E) 641log7−6(sint−cost)7+6(sint−cost)+C
›Reveal solutionSolution
Let u=sint−cost; the numerator becomes du and 13+36sin2t=49−36u2, giving a standard a2−k2u2du integral.
Set u=sint−cost. Then
du=(cost+sint)dt,
which is exactly the numerator, and
u2=1−2sintcost=1−sin2t⇒sin2t=1−u2.
So the denominator is
13+36sin2t=13+36(1−u2)=49−36u2.
Hence
∫49−36u2du=∫72−(6u)2du. …
- KEAM 2024Set eng-2024-06064 marksMCQQ.∫0π/4(tan3x+tan5x)dx= (A) 125 (B) 31 (C) 41 (D) 61 (E) 121
›Reveal solutionSolution
Factor out tan3xsec2x and substitute u=tanx to get 41.
tan3x+tan5x=tan3x(1+tan2x)=tan3xsec2x.
Let u=tanx, du=sec2xdx. Limits: x=0→u=0, x=4π→u=1. …
- 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. …
- KEAM 2026Set eng-2026-04184 marksMCQQ.∫(27x3(1−x3))32dx= (A) −43(1−x3)34+C (B) −53(1−x3)35+C (C) −39(1−x3)31+C (D) −49(1−x3)34+C (E) −59(1−x3)35+C
›Reveal solutionSolution
Simplify to 9x2(1−x3)2/3, then substitute u=1−x3 to obtain −59(1−x3)5/3+C.
Since (27x3(1−x3))2/3=272/3(x3)2/3(1−x3)2/3=9x2(1−x3)2/3, let u=1−x3, du=−3x2dx, so x2dx=−3du: …
- 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-2026-04184 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: …
🎓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.