Q.Evaluate the definite integral: ∫011−x2dx
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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)∣ …
The key idea is that the integrand 1−x21 is the derivative of sin−1x, so we can directly apply the standard formula.
Step 1: Recall the antiderivative:
∫1−x2dx=sin−1x+C
Step 2: Evaluate the definite integral from 0 to 1: …
The integral ∫011−x2dx is the area under the inverse sine derivative curve from 0 to 1, and its value is 2π.
Why This Integral Works
The expression 1−x21 is a classic derivative — it’s the derivative of sin−1x (or arcsinx). When you see an integrand that matches a known derivative form, the integration becomes immediate: you’re just reversing the differentiation. The limits 0 to 1 are especially nice because sin−1x is defined on [−1,1], and at x=1, it hits 2π.
The key insight: recognise the derivative pattern. No substitution is needed here — it’s a direct antiderivative. But if you wanted to use u-substitution, you could set x=sinu, which transforms the integral into ∫0π/2du, giving the same result.
Step-by-Step Solution
- Identify the antiderivative The integrand 1−x21 is the derivative of sin−1x (also written as arcsinx). So the indefinite integral is:
∫1−x2dx=sin−1x+C
- Apply the limits of integration Using the Fundamental Theorem of Calculus:
∫011−x2dx=sin−1(1)−sin−1(0)
- Evaluate the inverse sine values sin−1(1)=2π (since sin(2π)=1) sin−1(0)=0 (since sin(0)=0) …
Method: Definite integral giving an inverse sine
Recognise ∫1−x2dx=sin−1x, then evaluate across the limits.
Steps
Step 1: Match the standard form. 1−x21 integrates to sin−1x (not a logarithm).
Step 2: Bracket it.
∫ab1−x2dx=[sin−1x]ab. …
Common Mistakes
Mistake 1: Confusing 1−x21 with 1+x21 (arctan).
Why it's wrong: the radical-in-denominator form is arcsine; the 1+x2 form is arctangent. Correct approach: 1−x2⇒sin−1x.
Mistake 2: Using sin−11=π or 90 (degrees).
Why it's wrong: sin−11=2π in radians. Correct approach: work in radians for calculus. …
Showing the 12 most recent of 51 on this concept.
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.∫x2x4+x2+1x4−1dx= (A) x2x4+x2+1+c (B) xx4+x2+1+c (C) 2xx4+x2+1+c (D) x4x4+x2+1+c
›Reveal solutionSolution
Differentiating the candidate xx4+x2+1 reproduces the given integrand exactly, confirming it as the antiderivative.
Concept and Intuition
When an integrand looks like it could come from a quotient rule (a square root over a power of x), it is often faster to differentiate a plausible candidate of that shape and check, rather than search for a substitution from scratch.
Step-by-Step Solution
- Try g(x)=xx4+x2+1=xN where N=x4+x2+1.
- N′=2x4+x2+14x3+2x=Nx(2x2+1).
- Quotient rule: g′(x)=x2N′x−N=x2Nx2(2x2+1)−N=Nx2x2(2x2+1)−N2.
- N2=x4+x2+1, so the numerator is x2(2x2+1)−(x4+x2+1)=2x4+x2−x4−x2−1=x4−1.
- So g′(x)=x2x4+x2+1x4−1 — exactly the given integrand. …
- AP EAPCET 2025Set eng-2025-05-27-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
Substituting t=x turns the surd-heavy integrand into ∫(1+t)3/2(1−t)1/22dt, whose antiderivative is exactly −21+t1−t.
Concept and Intuition
When an integrand mixes x and x−x2=x1−x, substituting t=x clears every square root of x at once, converting the whole thing into a rational-power integral in t that matches the derivative of 1+t1−t — a standard "recognise the derivative" pattern worth memorising for CET-style problems.
Step-by-Step Solution
- Write x−x2=x(1−x)=x1−x, so the integral is
I=∫(1+x)x1−xdx.
- Let t=x, so x=t2, dx=2tdt:
I=∫(1+t)⋅t⋅1−t22tdt=∫(1+t)(1−t)(1+t)2dt=∫(1+t)3/2(1−t)1/22dt.
- Let y=1+t1−t. Differentiating y2=1+t1−t: 2yy′=(1+t)2−(1+t)−(1−t)=(1+t)2−2 ⇒ y′=y(1+t)2−1=(1+t)3/2(1−t)1/2−1. …
- 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 2022Set eng-2022-07-08-FN1 markMCQQ.0<x<1, ∫x2−x5dx=31log∣f(x)∣+C, then f(1/2)= (A) 8+78−7 (B) 8−78+7 (C) 2(8−7) (D) 2(8−7)2
›Reveal solutionSolution
Substituting t=x3 then 1−t=w2 integrates ∫dx/(x1−x3) cleanly to 31log1+1−x31−1−x3, and evaluating at x=1/2 gives 8+78−7.
Concept and Intuition
The key simplification is x2−x5=x2(1−x3), since 0<x<1 makes x>0 so x2(1−x3)=x1−x3. From there, the substitution t=x3 turns the integral into the very standard form ∫t1−tdt, solvable by a further substitution 1−t=w2.
Step-by-Step Solution
- Rewrite: I=∫x2−x5dx=∫x1−x3dx (using x>0).
- Let t=x3⇒dt=3x2dx⇒dx=3x2dt. Then I=∫3x2⋅x1−tdt=∫3x31−tdt=31∫t1−tdt (since x3=t).
- Let 1−t=w2⇒t=1−w2, dt=−2wdw: ∫t1−tdt=∫(1−w2)w−2wdw=−2∫1−w2dw=−log1−w1+w=log1+w1−w.
- So I=31log1+w1−w+C where w=1−t=1−x3, i.e. f(x)=1+1−x31−1−x3.
- Verify by differentiating (chain rule through w) that this reproduces x1−x31 — confirmed. …
- 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 2025Set eng-2025-05-22-AN1 markMCQQ.If ∫sin2x+sin4xcos3xdx=c−cosecx−f(x), then f(2π)= (A) 1 (B) 0 (C) 2π (D) π
›Reveal solutionSolution
Substituting s=sinx and partial-fractioning gives −cosecx−2Tan−1(sinx), so f(x)=2Tan−1(sinx) and f(π/2)=π/2.
Concept and Intuition
Writing cos3xdx=cos2x⋅cosxdx=(1−sin2x)d(sinx) converts a trig integral into an algebraic one in s=sinx, which is then handled by ordinary partial fractions.
Step-by-Step Solution
- Let s=sinx, so ds=cosxdx, and cos3xdx=(1−s2)ds.
- The integral becomes ∫s2+s41−s2ds=∫s2(1+s2)1−s2ds.
- Partial fractions (in u=s2): u(1+u)1−u=u1−1+u2, so the integrand is s21−1+s22.
- Integrating: ∫(s21−1+s22)ds=−s1−2Tan−1s+C=−cosecx−2Tan−1(sinx)+C. …
- AP EAPCET 2024Set eng-2024-05-21-FN1 markMCQQ.If n≥2 is a natural number and 0<θ<2π, then ∫cosn+1θ(cosnθ−cosθ)1/nsinθdθ= (A) n−1n(cos(1−n)θ−1)2+c (B) (n+1)(1−n)n(cos(1−n)θ−1)1+n1+c (C) n−11(cos(n−1)θ−1)2+c (D) 1−n2n(1−cos(1−n)θ)(n+1)/n
›Reveal solutionSolution
Factor out cosnθ from inside the radical to isolate a clean power of cosθ, then substitute w=cos1−nθ to reduce the whole integral to ∫w1/ndw.
Concept and Intuition
The key move is factoring cosnθ−cosθ=cosnθ(1−cos1−nθ) so the n-th root pulls a clean cosθ outside, cancelling nicely against the cosn+1θ in the denominator and leaving a single power of w=cos1−nθ whose differential exactly matches sinθdθ/cosnθ in the integrand.
Step-by-Step Solution
- Factor: cosnθ−cosθ=cosnθ(1−cos1−nθ), so
(cosnθ−cosθ)1/n=cosθ(1−cos1−nθ)1/n.
- Divide by cosn+1θ:
cosn+1θ(cosnθ−cosθ)1/n=cosnθ(1−cos1−nθ)1/n.
- Let w=cos1−nθ. Then dθdw=(1−n)cos−nθ⋅(−sinθ)=(n−1)sinθcos−nθ, so cosnθsinθdθ=n−1dw.
- The whole integrand times dθ becomes (1−w)1/n⋅n−1dw — wait, more directly w1/n is the factor (1−cos1−nθ)1/n once we track 1−cos1−nθ as the base; carrying the substitution through consistently: …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.∫(x5+1)6/5dx= (A) 5x5+11+c (B) x5x5+1+c (C) 5x5+1x+c (D) 5x5+1+c
›Reveal solutionSolution
Recognizing the integrand as the derivative of 5x5+1x (verified by direct differentiation) gives the antiderivative immediately.
Concept and Intuition
For integrals of the form ∫(xn+1)(n+1)/ndx, a useful trick is to guess that the antiderivative looks like (xn+1)1/nx (a ratio designed to make the product-rule differentiation collapse nicely), and then verify by differentiating it — if it reproduces the integrand exactly, we're done. This is often faster than a substitution for this particular family.
Step-by-Step Solution
- Guess the antiderivative g(x)=(x5+1)1/5x=x(x5+1)−1/5.
- Differentiate using the product rule: g′(x)=(x5+1)−1/5+x⋅(−51)(x5+1)−6/5⋅5x4.
- Simplify the second term: x⋅(−51)(5x4)(x5+1)−6/5=−x5(x5+1)−6/5.
- So g′(x)=(x5+1)−1/5−x5(x5+1)−6/5.
- Factor out (x5+1)−6/5: g′(x)=(x5+1)−6/5[(x5+1)−x5]=(x5+1)−6/5×1=(x5+1)−6/5. …
- 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 2021Set eng-2021-08-25-AN1 markMCQQ.∫cosxdx= (A) 2xsinx+2cosx+c (B) 2xsinx+2sinx+c (C) 2xsinx−2cosx+c (D) xcosx−2sinx+c
›Reveal solutionSolution
Substitute t=x to turn the integral into a standard integration-by-parts problem. Answer: 2xsinx+2cosx+c.
Concept and Intuition
Whenever you see x trapped inside a trig or exponential function, substituting t=x converts it into a polynomial-times-trig integral solvable by parts.
Step-by-Step Solution
- Let t=x⇒x=t2, dx=2tdt.
- ∫cosxdx=∫cost⋅2tdt=2∫tcostdt.
- Integrate by parts: ∫tcostdt=tsint−∫sintdt=tsint+cost. …
- 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 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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