Q.Evaluate the definite integral: ∫02/34+9x2dx equals (A) 6π (B) 12π (C) 24π (D) 4π
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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)∣ …
Concept: U Substitution — the integrand matches the form a2+u21, whose antiderivative is a1tan−1au.
Step 1: Write the integral as
∫02/34+9x2dx=∫02/322+(3x)2dx.
Step 2: Let u=3x, so du=3dx and dx=du/3. When x=0, u=0; when x=2/3, u=2.
∫02/34+9x2dx=31∫0222+u2du.
Step 3: Use the standard formula ∫a2+u2du=a1tan−1au with a=2: …
The integral ∫02/34+9x2dx is solved by recognizing the form a2+u21 and using the substitution u=3x, leading to the result 24π, which corresponds to option (C).
The key to this problem is spotting that the denominator 4+9x2 looks like a2+u2, the classic form for an inverse tangent integral. The standard formula is:
∫a2+u2du=a1tan−1(au)+C
Here, 4 is 22, and 9x2 is (3x)2. So we have a=2 and u=3x. The substitution u=3x will cleanly match the formula.
Let’s work through it step by step.
-
Set up the substitution.
Let u=3x. Then du=3dx, so dx=3du.
The limits change: when x=0, u=0; when x=32, u=3⋅32=2.
-
Rewrite the integral.
Substitute everything into the integral:
∫02/34+9x2dx=∫u=0u=24+u2du/3=31∫0222+u2du
- Apply the inverse tangent formula. Using ∫a2+u2du=a1tan−1(au) with a=2:
31[21tan−1(2u)]02=61[tan−1(2u)]02
- Evaluate the limits. At u=2: tan−1(22)=tan−1(1)=4π. At u=0: tan−1(0)=0. So the result is: 61(4π−0)=24π …
Method: Reduce a2+u21 by a linear substitution to the arctan standard form
When a quadratic denominator is a sum of two squares with a coefficient on x, factor it as a2+(kx)2 and substitute u=kx to reach the memorised inverse-tangent integral.
Steps
Step 1: Write the denominator as a sum of squares.
Recognise A+Bx2=(A)2+(Bx)2, i.e. a2+u2 with a=A and u=Bx.
Step 2: Substitute u=Bx. …
Common Mistakes
Mistake 1: Ignoring the coefficient of x2 and treating the denominator as 22+x2.
Why it's wrong: 4+9x2=22+(3x)2, so the "u" is 3x, not x; missing this scales the answer wrongly. Correct approach: substitute u=3x, giving the extra 31 from dx=3du.
Mistake 2: Forgetting the a1 in the arctan formula. …
Showing the 12 most recent of 51 on this concept.
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.If ∫2cosx+3sinx+4dx=32f(x)+c, then f(32π)= (A) 12π (B) 8π (C) 125π (D) 85π
›Reveal solutionSolution
This is a Weierstrass (t=tan(x/2)) substitution problem for a linear combination of sine and cosine plus a constant in the denominator. Evaluating f at the given point gives 125π, option (C).
Concept and Intuition
Whenever the denominator mixes sinx, cosx and a constant, the universal substitution t=tan(x/2) (with cosx=1+t21−t2, sinx=1+t22t, dx=1+t22dt) converts the trigonometric denominator into a plain quadratic in t, reducing the whole problem to a standard ∫quadraticdt that integrates to an arctangent.
Step-by-Step Solution
- Substitute: denominator becomes
2⋅1+t21−t2+3⋅1+t22t+4=1+t22−2t2+6t+4+4t2=1+t22t2+6t+6.
- The integral becomes ∫(2t2+6t+6)/(1+t2)2dt/(1+t2)=∫2t2+6t+62dt=∫t2+3t+3dt.
- Complete the square: t2+3t+3=(t+23)2+43, so ∫(t+23)2+43dt=3/21arctan(3/2t+3/2)+c=32arctan(32t+3)+c. …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.∫cos6x+sin6xsin2xcos2xdx= (A) 21Tan−1(tan2x)+c (B) 31Tan−1(tan2x)+c (C) 31Tan−1(tan3x)+c (D) Tan−1(tan3x)+c
›Reveal solutionSolution
Dividing through by cos6x to introduce tanx, then a double substitution (t=tanx, then u=t3), reduces the integral to a standard arctangent form, giving 31Tan−1(tan3x)+c.
Concept and Intuition
When an integrand has sin and cos appearing only in even powers that can be grouped, dividing everything by the highest power of cosx converts the whole expression into a rational function purely of tanx — a very common trick that opens the door to the substitution t=tanx. Here, since the denominator naturally forms 1+tan6x, a further substitution u=t3 turns it into the standard ∫1+u2du arctangent integral.
Step-by-Step Solution
- Divide numerator and denominator of cos6x+sin6xsin2xcos2x by cos6x: numerator becomes cos6xsin2xcos2x=cos4xsin2x=tan2xsec2x; denominator becomes 1+tan6x.
- The integral is now ∫1+tan6xtan2xsec2xdx.
- Substitute t=tanx, so dt=sec2xdx. The integral becomes ∫1+t6t2dt.
- Substitute u=t3, so du=3t2dt, i.e. t2dt=3du. The integral becomes 31∫1+u2du. …
- AP EAPCET 2023Set eng-2023-05-15-AN1 markMCQQ.∫4+5cosxdx= (A) −31log3−tan2x3+tan2x+C (B) 31log3−tan2x3+tan2x+C (C) −91log3+tan2x3−tan2x+C (D) 91log3+tan2x3−tan2x+C
›Reveal solutionSolution
The Weierstrass substitution t=tan(x/2) turns this into a standard ∫dt/(a2−t2) integral, giving option (B).
Concept and Intuition
For ∫a+bcosxdx the substitution t=tan(x/2) (so cosx=1+t21−t2, dx=1+t22dt) always converts the integral into a rational function of t alone.
Step-by-Step Solution
- cosx=1+t21−t2, dx=1+t22dt, with t=tan(x/2).
- 4+5cosx=4+5⋅1+t21−t2=1+t24(1+t2)+5(1−t2)=1+t29−t2.
- Integral becomes ∫(9−t2)/(1+t2)2dt/(1+t2)=∫9−t22dt.
- Using ∫a2−t2dt=2a1loga−ta+t+C with a=3: ∫9−t2dt=61log3−t3+t+C. …
- AP EAPCET 2021Set eng-2021-08-23-AN1 markMCQQ.∫2/e1/ex(logx)1/31dx= (A) 23{1+(log(2)−1)2/3} (B) 1 (C) 23{1+(log(2)+1)3/2} (D) 23{1−(log(2)−1)2/3}
›Reveal solutionSolution
Substitute u=logx to turn the integral into ∫u−1/3du, then evaluate between the transformed limits u=log2−1 and u=−1.
Concept and Intuition
An integrand of the form x⋅g(logx)1 always calls for the substitution u=logx, since du=dx/x removes the x and 1/x entirely, leaving a pure power of u.
Step-by-Step Solution
- Let u=logx, so du=dx/x.
- Limits: at x=2/e, u=log(2/e)=log2−1. At x=1/e, u=log(1/e)=−1.
- The integral becomes ∫log2−1−1u−1/3du=[2/3u2/3]log2−1−1=23[u2/3]log2−1−1.
- Using the real cube root, (−1)2/3=((−1)1/3)2=(−1)2=1.
- So the value is 23[1−(log2−1)2/3].
Common Mistakes …
- AP EAPCET 2026Set eng-2026-05-13-AN1 markMCQQ.∫x2(x4+1)3/4dx= (A) (1+x41)3/4+c (B) (1+x61)1/2+c (C) −(1+x41)−1/4+c (D) −(1+x41)1/4+c
›Reveal solutionSolution
Pulling x4 out from under the radical and substituting t=1+x−4 reduces the integral to a simple power rule, giving −(1+1/x4)1/4+c.
Concept and Intuition
When the integrand has (xn+1)p, it often helps to factor out the highest power of x from inside the bracket so that a substitution like t=1+x−n produces a clean differential matching the rest of the integrand.
Step-by-Step Solution
- Write (x4+1)3/4=(x4(1+x41))3/4=x3(1+x41)3/4.
- So x2(x4+1)3/41=x2⋅x3(1+x41)3/41=x5(1+x41)3/41=x−5(1+x−4)−3/4.
- Let t=1+x−4. Then dt=−4x−5dx, so x−5dx=−4dt. …
- AP EAPCET 2023Set eng-2023-05-17-AN1 markMCQQ.If ∫1+x2x3dx=A(1+x2)3/2+B(1+x2)1/2+C, then A+B= (A) 2/3 (B) −2/3 (C) 1/3 (D) −1/3
›Reveal solutionSolution
The substitution u=1+x2 turns the integral into a simple power-rule computation, giving A=1/3 and B=−1, so A+B=−2/3.
Concept and Intuition
Whenever the integrand has an odd power of x alongside a function of x2 (here 1+x2), substituting u=1+x2 (so du=2xdx) converts the odd-power part into a polynomial in u, making the integral elementary.
Step-by-Step Solution
- Let u=1+x2, du=2xdx, and x2=u−1.
- x3dx=x2⋅xdx=(u−1)⋅2du.
- ∫1+x2x3dx=∫u(u−1)⋅2du=21∫(u1/2−u−1/2)du.
- =21(32u3/2−2u1/2)+C=31u3/2−u1/2+C. …
- AP EAPCET 2021Set eng-2021-10-05-FN1 markMCQQ.∫sin3xsinxdx= (A) 231log3−tanx3+tanx+c (B) 231log3+tanx3−tanx+c (C) 431log3−tanx3+tanx+c (D) 431log3+tanx3−tanx+c
›Reveal solutionSolution
Use the triple-angle identity to cancel sinx, rewrite in terms of cos2x, then apply the Weierstrass-type substitution t=tanx to reduce to a standard rational integral. The answer is (A).
Concept and Intuition
sin3x factors as sinx(3−4sin2x), so sinx cancels immediately with the numerator, turning a trigonometric-looking integral into a much simpler one in sin2x (hence in cos2x), which is a textbook target for the t=tanx substitution.
Step-by-Step Solution
- sin3x=3sinx−4sin3x=sinx(3−4sin2x).
- sin3xsinx=3−4sin2x1.
- Using sin2x=21−cos2x: 4sin2x=2−2cos2x, so 3−4sin2x=1+2cos2x.
- Integral becomes ∫1+2cos2xdx.
- Substitute t=tanx, dx=1+t2dt, cos2x=1+t21−t2: 1+2cos2x=1+t2(1+t2)+2(1−t2)=1+t23−t2.
- Integral =∫3−t21+t2⋅1+t2dt=∫3−t2dt. …
- AP EAPCET 2026Set eng-2026-05-12-FN1 markMCQQ.∫log2log3e2x−1e3x−3exdx= (A) log(32e) (B) log(34e) (C) 32e (D) 34e
›Reveal solutionSolution
A substitution t=ex turns this exponential integral into a rational one; the value works out to log(2e/3).
Concept and Intuition
Whenever an integral is built entirely from powers of ex, substituting t=ex converts it into an algebraic (rational function) integral, which is usually far easier to handle with partial fractions.
Step-by-Step Solution
- Let t=ex, so dt=exdx=tdx, i.e. dx=dt/t. When x=log2, t=2; when x=log3, t=3.
- Rewrite the integrand: e2x−1e3x−3ex=t2−1t3−3t. Multiplying by dx=dt/t gives t(t2−1)t3−3tdt=t2−1t2−3dt.
- So the integral becomes ∫23t2−1t2−3dt.
- Split: t2−1t2−3=t2−1(t2−1)−2=1−t2−12. …
- AP EAPCET 2022Set eng-2022-07-06-AN1 markMCQQ.∫cos4xcos2x1dx=421log(1−f(x)1+f(x))−21logg(x)+C, then g(6π)−2f(6π)= (A) 22π (B) π+3 (C) 2 (D) 1
›Reveal solutionSolution
Solving the integral via t=sin2x identifies f(x)=2sin2x and g(x)=1−sin2x1+sin2x; evaluating at x=π/6 gives g(π/6)−2f(π/6)=2.
Concept and Intuition
The integral ∫cos4xcos2xdx is tackled by substituting t=sin2x, since cos4x=1−2sin22x=1−2t2 turns the whole integrand into a rational function of t, solvable by partial fractions into two logarithmic terms — one built from 1−t2 and one from 1−2t2, matching exactly the two-log structure given in the problem.
Step-by-Step Solution
- Let t=sin2x, so dt=2cos2xdx and cos4x=1−2t2.
- Rewriting the integral in terms of t: ∫cos4xcos2xdx=∫2(1−t2)(1−2t2)dt.
- Partial fractions: (1−t2)(1−2t2)1=1−t2−1+1−2t22.
- Integrating each piece gives standard log forms: one in 1−t1+t (from the 1−t2 term) and one in 1−2t1+2t (from the 1−2t2 term), exactly matching the pattern 421log1−f1+f−21logg with f(x)=2sin2x and g(x)=1−sin2x1+sin2x.
- Evaluate at x=π/6: 2x=π/3, sin(π/3)=23. …
- AP EAPCET 2023Set eng-2023-05-18-FN1 markMCQQ.If ∫x(1−x3)2−1dx=32g(f(x))+c, then (A) f(x)=x, g(x)=sin−1x (B) f(x)=x3/2, g(x)=sin−1x (C) f(x)=x3/2, g(x)=cos−1x (D) f(x)=x, g(x)=cos−1x
›Reveal solutionSolution
A substitution u=x3/2 turns the integral into the standard ∫du/1−u2 form, giving f(x)=x3/2 and g=sin−1.
Concept and Intuition
The presence of xdx alongside x3=(x3/2)2 inside a square root strongly signals the substitution u=x3/2 (its derivative is proportional to x, exactly what's needed to absorb the leftover xdx). Once substituted, the integral collapses to the standard arcsine form.
Step-by-Step Solution
- Let u=x3/2. Then du=23x1/2dx=23xdx, so xdx=32du.
- Also, u2=x3, so 1−x3=1−u2.
- Substitute into the integral: ∫x(1−x3)−1/2dx=∫32⋅1−u2du=32∫1−u2du.
- This is the standard form: ∫1−u2du=sin−1u+c.
- So the integral =32sin−1(u)+c=32sin−1(x3/2)+c. …
- AP EAPCET 2026Set eng-2026-05-15-FN1 markMCQQ.∫(1+sinx)4cos3xdx= (A) −5(1+sinx)5cos4x+c (B) 5(1+sinx)5cos4x+c (C) 4(1+sinx)4cos4x+c (D) −4(1+sinx)4cos4x+c
›Reveal solutionSolution
Factor cos3x using cos2x=(1−sinx)(1+sinx) and substitute t=sinx; the resulting antiderivative can equivalently be written in the cos4x/(1+sinx)4 form given in the options (they differ only by an added constant). Answer: −4(1+sinx)4cos4x+c.
Concept and Intuition
Integrals of cosoddx over powers of (1+sinx) are handled by peeling off one factor of cosx to pair with dx (making d(sinx)) and expressing the remaining even power of cosx in terms of sinx. Since the MCQ options are phrased in terms of cos4x rather than sinx directly, it is often faster (and safer against sign traps) to guess-and-check an antiderivative of that shape by differentiating a general form Acos4x(1+sinx)−n and matching powers/coefficients — this is exactly how the printed option is confirmed.
Step-by-Step Solution
- cos3x=cosx⋅cos2x=cosx(1−sin2x)=cosx(1−sinx)(1+sinx).
- Integrand =(1+sinx)4cosx(1−sinx)(1+sinx)=(1+sinx)3cosx(1−sinx).
- Let t=sinx, dt=cosxdx: I=∫(1+t)31−tdt. Writing 1−t=2−(1+t): I=∫((1+t)32−(1+t)21)dt=−(1+t)21+1+t1+c=(1+t)2t+c.
- So I=(1+sinx)2sinx+c is one valid closed form. …
- AP EAPCET 2022Set eng-2022-07-05-FN1 markMCQQ.∫(x−3)4/5(x+1)6/5dx= (A) 455x+1x−3+C (B) 45(x−3x+1)1/5+C (C) 51(x+1x−3)1/5+C (D) 45(x+4x−3)4/5+C
›Reveal solutionSolution
Splitting off (x+1)2 turns the integrand into a pure power of t=x+1x−3, giving 45(x+1x−3)1/5+C.
Concept and Intuition
When an integrand has the form (x−a)p(x−b)q with p+q an integer (here 54+56=2), factoring out (x−b)p+q and substituting t=x−bx−a collapses the whole thing to a simple power of t — a standard trick for these "unequal fractional exponent" integrals.
Step-by-Step Solution
- (x−3)4/5(x+1)6/5=(x+1)2[x+1x−3]4/5 (factoring out (x+1)4/5+6/5=(x+1)2).
- So the integrand is (x+1)−2[x+1x−3]−4/5.
- Let t=x+1x−3. Then dxdt=(x+1)2(x+1)−(x−3)=(x+1)24, so (x+1)−2dx=4dt.
- The integral becomes 41∫t−4/5dt=41⋅1/5t1/5+C=45t1/5+C. …
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