Q.Integrate the function 1+9x2
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Integration By Completing Square
Integration by Completing the Square
You can integrate x2+11 or x2−a21 on sight. But a quadratic denominator such as x2+4x+5 fits neither standard form directly. The fix is to rewrite the quadratic as a perfect square plus (or minus) a constant, turning it into a form you already know.
The move
For x2+bx+c, add and subtract (2b)2:
x2+bx+c=(x+2b)2+(c−4b2).
After substituting u=x+2b the integral collapses to ∫u2+k2du or ∫u2−k2du.
Case A — leads to inverse tangent
∫x2+4x+5dx.
Complete the square: x2+4x+5=(x+2)2+1. With u=x+2,
∫u2+1du=tan−1u+C=tan−1(x+2)+C.
∫u2+a2du=a1tan−1au+C is the workhorse when the constant left over is positive.
Case B — leads to a logarithm
∫x2−6x+5dx.
Here x2−6x+5=(x−3)2−4. With u=x−3 the denominator u2−4 factors, so use partial fractions:
∫u2−4du=41logu+2u−2+C=41logx−1x−5+C. …
The key idea is to rewrite the integrand in a form that matches the standard formula for ∫a2+x2dx, using completing the square (here, the square is already complete).
First, simplify the square root:
1+9x2=99+x2=31x2+9.
Now integrate:
∫1+9x2dx=31∫x2+9dx.
Use the standard formula ∫x2+a2dx=2xx2+a2+2a2sinh−1ax+C (or the equivalent logarithmic form). Here a=3, so: …
We integrate 1+9x2 by rewriting it as 31x2+9, then using the standard trigonometric substitution x=3tanθ. The final result is 6x1+9x2+23sinh−1(3x)+C, or equivalently 6x1+9x2+23logx+x2+9+C.
The expression 1+9x2 looks like it came straight from a right triangle. When you see 1+(something)2 under a square root, your mind should immediately go to one of two places: either a trigonometric substitution (like x=atanθ) or a hyperbolic substitution (like x=asinht). Both work; the choice is a matter of taste.
The key insight: the constant 1 and the fraction 9x2 are not in the simplest form for substitution. Factor out the 91 first.
- Simplify the integrand algebraically
1+9x2=99+x2=3x2+9
So the integral becomes:
I=∫1+9x2dx=31∫x2+9dx
Now we have a clean x2+a2 form with a=3.
-
Choose the substitution
For x2+a2, the standard trigonometric substitution is x=atanθ. Why? Because 1+tan2θ=sec2θ, which turns the square root into something simple.
Let x=3tanθ. Then dx=3sec2θdθ.
Watch outA common mistake: forgetting to also change dx when substituting. The dx is not dθ — you must multiply by the derivative.
-
Rewrite the integrand in θ
x2+9=9tan2θ+9=9(tan2θ+1)=3sec2θ=3∣secθ∣
For the principal range of θ=tan−1(x/3), we have θ∈(−π/2,π/2), where secθ>0. So we can drop the absolute value: x2+9=3secθ.
Therefore:
I=31∫(3secθ)⋅(3sec2θdθ)=31∫9sec3θdθ=3∫sec3θdθ
-
Integrate sec3θ
This is a classic integral. The trick: write sec3θ=secθ⋅sec2θ and integrate by parts.
Let u=secθ, dv=sec2θdθ. Then du=secθtanθdθ, v=tanθ.
∫sec3θdθ=secθtanθ−∫secθtan2θdθ
Now use tan2θ=sec2θ−1:
∫sec3θdθ=secθtanθ−∫secθ(sec2θ−1)dθ
=secθtanθ−∫sec3θdθ+∫secθdθ
Bring the ∫sec3θ term to the left:
2∫sec3θdθ=secθtanθ+∫secθdθ
And ∫secθdθ=log∣secθ+tanθ∣+C.
So:
∫sec3θdθ=21secθtanθ+21log∣secθ+tanθ∣+C
Memorise the result for ∫sec3θdθ — it appears often in integrals involving x2+a2.
-
Substitute back to x
We have I=3∫sec3θdθ, so:
I=3[21secθtanθ+21log∣secθ+tanθ∣]+C
I=23secθtanθ+23log∣secθ+tanθ∣+C
Now recall: x=3tanθ, so tanθ=3x. …
Method: Factor out the constant, then use a standard t2+a2 formula
When the quadratic under the root has a fractional or non-unit coefficient, e.g. 1+9x2, pull the constant out of the root to expose a clean x2+a2 form.
Steps
Step 1: Combine into a single fraction under the root.
1+9x2=99+x2=319+x2.
Step 2: Identify a2.
Now it is 31x2+a2 with a2=9, a=3.
Step 3: Apply the standard integral. …
Common Mistakes
Mistake 1: Pulling the constant out as 91 instead of 31.
Why it's wrong: 99+x2=919+x2=319+x2 — the root of 9 is 3. Correct approach: take the square root of the denominator.
Mistake 2: Using a2=3. …
Showing the 12 most recent of 35 on this concept.
- AP EAPCET 2025Set eng-2025-05-22-AN1 markMCQQ.∫x2+x+1dx (A) 4(2x+1)x2+x+1+83Sinh−1(32x+1)+c (B) 4x+1x2+x+1+83Sinh−1(32x+1)+c (C) 4x+1x2+x+1−83Sinh−1(32x+1)+c (D) 4(2x+1)x2+x+1−83Sinh−1(32x+1)+c
›Reveal solutionSolution
Completing the square turns x2+x+1 into the standard u2+a2 form, whose known integral gives option (A).
Concept and Intuition
The standard result ∫u2+a2du=2uu2+a2+2a2Sinh−1(au)+c applies to any quadratic under a square root once it's written as a perfect square plus a constant.
Step-by-Step Solution
- Complete the square: x2+x+1=(x+21)2+43. Let u=x+21, a2=43 (so a=23).
- Apply the formula: ∫u2+a2du=2uu2+a2+2a2Sinh−1(au)+c.
- 2u=2x+21=42x+1, and u2+a2=x2+x+1.
- 2a2=23/4=83, and au=3/2x+21=32x+1. …
- AP EAPCET 2024Set eng-2024-05-18-FN1 markMCQQ.∫x2+x+1x+1dx= (A) 21x2+x+1+21cosh−1(3x+2)+c (B) 21x2+x+1+32tan−1(32x+1)+c (C) x2+x+1+32log∣x2+x+1∣+c (D) x2+x+1+21sinh−1(32x+1)+c
›Reveal solutionSolution
Splitting the numerator into a multiple of the derivative of the radicand plus a constant is the standard technique for ∫ax2+bx+cpx+qdx; here it gives x2+x+1+21sinh−1(32x+1)+c.
Concept and Intuition
For ∫quadraticlineardx, write the linear numerator as A⋅(derivative of quadratic)+B. The A-part becomes a simple power-rule integral (since it's exactly u−1/2du), and the B-part reduces to the standard ∫x2+a2dx=sinh−1(x/a)+c form after completing the square.
Step-by-Step Solution
- Write x+1=21(2x+1)+21.
- First part: 21∫x2+x+12x+1dx. Let u=x2+x+1, du=(2x+1)dx: this is 21∫u−1/2du=21⋅2u1/2=x2+x+1.
- Second part: 21∫x2+x+1dx=21∫(x+1/2)2+3/4dx.
- This is of the form 21∫u2+a2du with u=x+1/2, a=3/2, giving 21sinh−1(au)=21sinh−1(32x+1). …
- AP EAPCET 2023Set eng-2023-05-16-FN1 markMCQQ.∫x32x4−2x2+1x2−1dx= (A) 2x212x4+2x2+1+C (B) 2x212x4−2x2+1+C (C) 2x214x4−2x2+1+C (D) 2x214x4+2x2+1+C
›Reveal solutionSolution
Verify by differentiation: F(x)=2x212x4−2x2+1 differentiates back to the given integrand, so this is the antiderivative.
Concept and Intuition
When an integrand looks like it could come from differentiating a quotient of the form x2quartic, the fastest rigorous route (especially under exam time pressure) is to differentiate the most plausible option and check it reproduces the integrand exactly, rather than deriving the substitution from scratch.
Step-by-Step Solution
- Let u=2x4−2x2+1 and F=2x2u.
- u′=2u8x3−4x=u4x3−2x.
- Quotient rule: F′=4x4u′⋅2x2−u⋅4x=4x4u(4x3−2x)⋅2x2−4xu.
- Multiply numerator and denominator by u: F′=4x4u2x2(4x3−2x)−4xu2=4x4u8x5−4x3−4x(2x4−2x2+1).
- Expand the numerator: 8x5−4x3−8x5+8x3−4x=4x3−4x. …
- AP EAPCET 2023Set eng-2023-05-17-AN1 markMCQQ.∫(2ax+x2)3/2dx= (A) a21(2ax+x2x+a)+C (B) a21(2ax+x2x−a)+C (C) a2−1(2ax+x2x−a)+C (D) a2−1(2ax+x2x+a)+C
›Reveal solutionSolution
Completing the square converts the integral into the standard form ∫du/(u2−a2)3/2, which has a known closed form; back-substituting gives option (D).
Concept and Intuition
Many integrals of the form ∫dx/(quadratic)3/2 become standard once the quadratic is completed to a perfect-square-minus-constant form; the resulting substitution u=x+a reduces it to a memorized/derivable antiderivative.
Step-by-Step Solution
- 2ax+x2=x2+2ax+a2−a2=(x+a)2−a2. Let u=x+a, du=dx.
- The integral becomes ∫(u2−a2)3/2du.
- Standard result (verifiable by differentiation): ∫(u2−a2)3/2du=a2u2−a2−u+C. Check: dud[a2u2−a2−u]=(u2−a2)3/21 ✓. …
- AP EAPCET 2021Set eng-2021-08-25-AN1 markMCQQ.∫7−6x−x2dx= (A) Sinh−1(4x+3)+c (B) log4x+3+c (C) Sin−1(4x+3)+c (D) 21Sin−1(4x+3)+c
›Reveal solutionSolution
Completing the square under the root reveals the standard form ∫a2−u2dx=sin−1(u/a). Answer: sin−1(4x+3)+c.
Concept and Intuition
A quadratic under a square root, when completed to the square, reveals which standard integral form applies: a2−(x−h)2 gives an arcsine, while (x−h)2+a2 or (x−h)2−a2 give hyperbolic-inverse/log forms.
Step-by-Step Solution
- 7−6x−x2=−(x2+6x−7)=−[(x+3)2−9−7]=−(x+3)2+16=16−(x+3)2.
- So the integral is ∫42−(x+3)2dx.
- Using ∫a2−u2du=sin−1(au)+c with u=x+3, a=4: …
- AP EAPCET 2024Set eng-2024-05-22-FN1 markMCQQ.∫x32x4−2x2+1x2−1dx (A) 2x21+2x2+2x4+c (B) 2x2(1+2x2+2x4)1/2+c (C) 2x21−2x2+2x4+c (D) 2x2(1−2x2+2x4)1/2+c
›Reveal solutionSolution
Verifying the antiderivative by differentiating each candidate is faster and safer than guessing a substitution; the derivative of option (D) reproduces the integrand exactly. Answer: option (D).
Concept and Intuition
When an integral has an awkward-looking algebraic form under a square root, and the options are all algebraic expressions (not transcendental), the fastest rigorous check is to differentiate the candidate answers and see which one reproduces the integrand — this avoids errors in choosing a substitution.
Step-by-Step Solution
- Let N(x)=1−2x2+2x4 (note this equals 2x4−2x2+1, the expression under the root in the integrand).
- Try f(x)=2x2N1/2=21N1/2x−2.
- Differentiate: f′(x)=4N1/2x2N′−x3N1/2, where N′=−4x+8x3=4x(2x2−1).
- First term becomes xN1/2(2x2−1). Combine both terms over the common denominator x3N1/2:
f′(x)=x3N1/2(2x2−1)x2−N.
- Numerator: (2x2−1)x2−N=(2x4−x2)−(1−2x2+2x4)=x2−1. …
- AP EAPCET 2024Set eng-2024-05-22-FN1 markMCQQ.∫1+x+x22dx= (A) 34tan−1(32x−1)+c (B) 34tan−1(32x+1)+c (C) 32tan−1(32x−1)+c (D) 32tan−1(32x+1)+c
›Reveal solutionSolution
Complete the square in the denominator and apply the standard ∫x2+a2dx=a1tan−1(x/a) form. Answer: option (B).
Concept and Intuition
Any irreducible quadratic ax2+bx+c in a denominator under a simple rational integrand can be handled by completing the square to reduce it to the standard u2+a2 form, whose antiderivative is a scaled arctangent.
Step-by-Step Solution
- Complete the square: 1+x+x2=(x+21)2+43=(x+21)2+(23)2.
- So the integral is 2∫(x+21)2+(23)2dx.
- Using ∫u2+a2du=a1tan−1(u/a) with u=x+21, a=23:
2⋅3/21tan−1(3/2x+1/2)=34tan−1(32x+1)+c.
Common Mistakes …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.∫9cos2x−24sinxcosx+16sin2x1dx= (A) 4(3cosx−4sinx)cosx+c (B) 4(3cosx−4sinx)sinx+c (C) 3cosx−4sinxcosx+c (D) 3cosx−4sinxsinx+c
›Reveal solutionSolution
Recognising the denominator as the perfect square (3cosx−4sinx)2 and substituting u=3−4tanx reduces the integral to a simple power rule, giving 4(3cosx−4sinx)cosx+c.
Concept and Intuition
When a trigonometric denominator has the form a2cos2x−2absinxcosx+b2sin2x, check whether it is a perfect square (acosx−bsinx)2 first — this instantly turns a messy-looking integral into ∫sec2(⋅)/(linear in tanx)2dx, solvable by substitution.
Step-by-Step Solution
- Observe 9cos2x−24sinxcosx+16sin2x=(3cosx−4sinx)2 (matches a2−2ab+b2 with a=3cosx, b=4sinx).
- So the integral is ∫(3cosx−4sinx)2dx.
- Divide numerator and denominator by cos2x: =∫(3−4tanx)2sec2xdx.
- Let u=3−4tanx, so du=−4sec2xdx, i.e. sec2xdx=−41du. …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.∫9x2−12x+1(3x−2)tan(9x2−12x+1)dx= (A) 31sec29x2−12x+1+c (B) 31sec2x+c (C) 21logsec9x2−12x+1+c (D) 31logsec9x2−12x+1+c
›Reveal solutionSolution
A double substitution — first u=9x2−12x+1, then w=u — turns the integral into a plain ∫tanwdw, giving 31log∣sec9x2−12x+1∣+c.
Concept and Intuition
When (3x−2) (half the derivative of 9x2−12x+1) sits outside a function of 9x2−12x+1, a chained substitution — first for the quadratic, then for its square root — collapses the whole expression to a single-variable standard integral.
Step-by-Step Solution
- Let u=9x2−12x+1. Then du=(18x−12)dx=6(3x−2)dx, so (3x−2)dx=6du.
- Integral becomes ∫utanu⋅6du=61∫utanudu.
- Let w=u, so dw=2udu, i.e. udu=2dw.
- Integral becomes 61∫tanw⋅2dw=31∫tanwdw=31(−log∣cosw∣)+c=31log∣secw∣+c. …
- AP EAPCET 2023Set eng-2023-05-16-AN1 markMCQQ.∫x3x4+x−4+2dx= (A) log∣x∣−4x41+C (B) log∣x∣+4x41+C (C) log∣x∣−x44+C (D) log∣x∣+x44+C
›Reveal solutionSolution
The expression under the square root is a perfect square, simplifying the integral to a sum of two elementary power terms.
Concept and Intuition
Recognize x4+x−4+2 as (x2+x−2)2 using the identity a2+b2+2ab=(a+b)2 with a=x2,b=x−2.
Step-by-Step Solution
- (x2+x−2)2=x4+2+x−4 — matches the expression under the square root exactly.
- x4+x−4+2=∣x2+x−2∣=x2+x21 (always positive for real x=0).
- Divide by x3: x3x2+1/x2=x1+x51. …
- AP EAPCET 2022Set eng-2022-07-08-AN1 markMCQQ.∫(x+12x−36+x−12x−36)dx= (A) 23x+C, ∀x (B) 34(x−3)3/2+C, ∀x (C) ⎩⎨⎧34(x−3)3/2+C,23x+C,x>63≤x≤6 (D) ⎩⎨⎧34(x−3)3/2+C,23x+C,3≤x≤6x>6
›Reveal solutionSolution
Substituting t=x−3 turns both nested radicals into perfect squares (t±3)2; the integrand collapses to a constant 23 on [3,6] and to 2x−3 for x>6, giving the piecewise antiderivative in (C).
Concept and Intuition
Nested radicals of the form x±linear in x often simplify to perfect squares under a substitution that removes the inner square root. Here the key is recognizing 12x−36 is a perfect multiple of (x−3).
Step-by-Step Solution
- Let t=x−3≥0 (valid for x≥3), so x=t2+3 and 12x−36=12t2⇒12x−36=23t.
- x+12x−36=t2+3+23t=(t+3)2⇒x+12x−36=t+3 (always non-negative).
- x−12x−36=t2+3−23t=(t−3)2⇒x−12x−36=∣t−3∣.
- Sum =(t+3)+∣t−3∣. If t≥3 (i.e. x−3≥3⇒x≥6): sum =2t=2x−3. If 0≤t<3 (i.e. 3≤x<6): sum =(t+3)+(3−t)=23. …
- AP EAPCET 2026Set eng-2026-05-18-FN1 markMCQQ.∫(1+x)x−x2dx= (A) −21−x1+x+c (B) −1+x1−x+c (C) −21+x1−x+c (D) 21−x1+x+c
›Reveal solutionSolution
Two successive substitutions (t=x, then w=(1−t)/(1+t)) collapse the integral to ∫−2dw; the answer is −21+x1−x+c.
Concept and Intuition
The presence of x inside (1+x) and inside x−x2=x1−x both suggest first substituting t=x. What remains — a rational-times-square-root expression in t symmetric under t→−t in a (1±t) sense — is the classic cue for the substitution w2=1+t1−t, which rationalizes everything at once.
Step-by-Step Solution
- Note x−x2=x(1−x), so x−x2=x1−x, and the integral is
∫(1+x)x1−xdx.
- Let t=x, so x=t2, dx=2tdt:
∫(1+t)t1−t22tdt=∫(1+t)(1−t)(1+t)2dt=∫(1+t)3/2(1−t)1/22dt.
- Let w=1+t1−t, so t=1+w21−w2 and dt=(1+w2)2−4wdw. One finds
1+t=1+w22,1−t=1+w22w2,
so
(1+t)3/2(1−t)1/2=(1+w2)24w.
- Substituting, …
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