Q.Evaluate the definite integral: ∫02x2+46x+3dx
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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)∣ …
Split the numerator so one piece is (a multiple of) the derivative of x2+4 (a log) and the other is a constant over x2+4 (an arctan).
x2+46x+3=x2+46x+x2+43.
Log part: ∫02x2+46xdx=3[log(x2+4)]02=3(ln8−ln4)=3ln2. …
Split into a log part and an arctan part: the value is 3ln2+83π.
Setting up
The denominator x2+4 suggests an arctangent, but the numerator 6x+3 is linear. The standard move is to break it into two pieces: one whose numerator is a multiple of dxd(x2+4)=2x (giving a logarithm) and one that is a constant over x2+4 (giving an arctangent).
x2+46x+3=x2+46x+x2+43.
1. The logarithm piece
With u=x2+4, du=2xdx, so 6xdx=3du; limits u:4→8:
∫02x2+46xdx=∫48u3du=3(ln8−ln4)=3ln2.
2. The arctangent piece …
Method: Splitting a linear-over-quadratic integrand into log and arctan parts
For ∫x2+a2px+qdx, separate the numerator into a multiple of the denominator's derivative (a logarithm) plus a constant (an inverse tangent).
Steps
Step 1: Split the numerator around dxd(x2+a2)=2x.
Write px+q=2p(2x)+q, so one part is a constant times 2x.
Step 2: Integrate the derivative part to a log. …
Common Mistakes
Mistake 1: Treating the whole numerator 6x+3 as the denominator's derivative.
Why it's wrong: dxd(x2+4)=2x, so only 6x (a multiple of 2x) gives a log; the constant 3 needs the arctan formula. Correct approach: split into x2+46x+x2+43. …
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›Reveal solutionSolution
Simplify cosx−cos3x=cosxsin2x and recognise the derivative of cos−1(cos3/2x).
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Centering at t=x+2 turns (x+1)(x+3) into t2−1.
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- GUJCET 2020Set 071 markMCQQ.∫cosxsinxcotxdx= ________ +C. (A) −2cotx (B) −2tanx (C) 2cotx (D) cotx1
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Substitute t=cotx.
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Since cosx=cotxsinx, we have sinxcosx=sin2xcotx, so
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sin2x=2sinxcosx turns the integral into 2∫ueudu=2eu(u−1).
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Substituting u=sinx gives ∫u13(1−u2)du=14sin14x−16sin16x, so A+B=1121.
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The x²⁰¹⁹ factor is (up to a constant) the derivative of the exponent x²⁰²⁰.
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Recognize the integrand as f(x)f′(x) where f(x)=sec−1x.
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›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.
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Split off sec2x and write the rest in terms of tanx.
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›Reveal solutionSolution
Substitute u=cosx to turn this into a standard ∫a2+b2u2du integral.
Let u=cosx, so du=−sinxdx, i.e. sinxdx=−du.
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=−231tan−1(32cosx)+C.
…
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›Reveal solutionSolution
Recognize that ex(1+x) is the derivative of x⋅ex, so substitute t=xex.
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Multiply through by ex to turn the denominator into 1+e2x, a standard arctan form.
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