Q.Integrate the following function: x+4x, x>0
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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 — we let u equal the expression inside the square root to simplify the integral.
Let u=x+4, so x=u−4 and dx=du. The integral becomes:
∫uu−4du=∫(u1/2−4u−1/2)du
Integrate term by term:
32u3/2−8u1/2+C
Substitute back u=x+4:
32(x+4)3/2−8x+4+C …
The key idea is to use the substitution u=x+4 to eliminate the square root, turning the integral into a sum of power functions. The final result is 32(x+4)3/2−8x+4+C.
Why substitution works here
When you see a square root of a linear expression like x+4, your first instinct should be to let that whole expression become a new variable. The reason is simple: the derivative of x+4 is just 1, so dx=du, and the square root becomes u, which is a clean power u1/2. The numerator x is just u−4, so the whole integrand becomes a combination of powers of u — and those are the easiest functions to integrate.
Let’s walk through it.
-
Set up the substitution
Let u=x+4. Then x=u−4, and dx=du. The integral becomes:
∫x+4xdx=∫uu−4du
-
Rewrite as a sum of powers
Split the fraction:
uu−4=uu−u4=u1/2−4u−1/2
So the integral is:
∫(u1/2−4u−1/2)du
-
Integrate term by term
Using the power rule ∫undu=n+1un+1+C for n=−1:
∫u1/2du=3/2u3/2=32u3/2
∫4u−1/2du=4⋅1/2u1/2=8u1/2
Putting them together:
32u3/2−8u1/2+C
-
Substitute back
Replace u with x+4: …
Method: Substitute a linear denominator and split into powers
Use this when x sits over a root of a linear expression, e.g. x+4x: substitute the bracket and rewrite x so every term becomes a power of u.
Steps
Step 1: Substitute the inside of the root.
Let u=x+4, so du=dx and x=u−4.
Step 2: Rewrite as a sum of powers.
x+4xdx=u1/2u−4du=(u1/2−4u−1/2)du. …
Common Mistakes
Mistake 1: Not re-expressing x as u−4.
Why it's wrong: after u=x+4 the numerator x must become u−4, otherwise the integral mixes variables. Correct approach: substitute x=u−4 so the integrand is u1/2u−4.
Mistake 2: Not splitting u1/2u−4 into separate powers. …
Showing the 12 most recent of 51 on this concept.
- 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. …
- 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-2022-07-07-AN1 markMCQQ.∫x+x2+2dx= (A) 23(x+x+2)3/2−2(x+x2+2)1/4+C (B) 31(x+x2+2)3/2−2(x+x2+2)1/4+C (C) (x+x2+2)−3/2−2(x+x2+2)−1/2+C (D) 3x+x2+2(x+x2+2)2−6+C
›Reveal solutionSolution
The substitution t=x+x2+2 rationalises the nested radical; the resulting antiderivative, written as a single fraction, matches option (D). Answer: option (D).
Concept and Intuition
Integrals containing x+x2+a2 are a classic signal to substitute t equal to that whole expression — it converts the awkward nested square root into simple powers of t, because x and x2+a2 can both be written as clean rational/linear functions of t.
Step-by-Step Solution
- Let t=x+x2+2. Then x2+2=t−x; squaring, x2+2=t2−2tx+x2⇒2=t2−2tx⇒x=2tt2−2.
- Then x2+2=t−x=t−2tt2−2=2t2t2−t2+2=2tt2+2.
- Differentiate t w.r.t. x: dxdt=1+x2+2x=x2+2x2+2+x=x2+2t=(t2+2)/(2t)t=t2+22t2, so dx=2t2t2+2dt.
- Substitute into the integral: ∫tdx=∫t1/2⋅2t2t2+2dt=21∫(t1/2+2t−3/2)dt.
- Integrate: 21[32t3/2−4t−1/2]+C=31t3/2−2t−1/2+C. …
- AP EAPCET 2024Set eng-2024-05-22-FN1 markMCQQ.∫1+x8x3tan−1x4dx= (A) 8(tan−1(x4))2+c (B) 3(tan−1(x4))3+c (C) 4(tan−1(x4))2+c (D) 2(tan−1(x4))2+c
›Reveal solutionSolution
A double substitution (first u=x4, then v=tan−1u) turns this into a trivial ∫vdv. Answer: 8(tan−1x4)2+c.
Concept and Intuition
The presence of x3dx alongside x4 inside the arctan and x8=(x4)2 in the denominator is a strong signal to substitute u=x4 first. After that, the structure tan−1u⋅1+u2du is exactly of the form "function times its own derivative," solved by a second substitution.
Step-by-Step Solution
- Let u=x4, so du=4x3dx⇒x3dx=4du.
- The integral becomes ∫1+u2tan−1u⋅4du=41∫1+u2tan−1udu. …
- 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 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 2025Set eng-2025-05-21-AN1 markMCQQ.If ∫(x−1)3/2(x−3)1/2dx=f(x)+c then f(−1)−f(0)= (A) −3 (B) −4 (C) −2 (D) −1
›Reveal solutionSolution
A classic "divide by (x−1)2 and substitute t=(x−3)/(x−1)" integral. It reduces to t, giving f(x)=(x−3)/(x−1), and then f(−1)−f(0)=−1.
Concept and Intuition
When an integrand has two linear factors under different fractional powers whose exponents sum to an integer (here 3/2+1/2=2), factor out (x−1)2 from the denominator and express the rest as a function of the ratio x−1x−3. This ratio then becomes a natural single substitution variable.
Step-by-Step Solution
- Write the denominator as
(x−1)3/2(x−3)1/2=(x−1)2⋅(x−1)1/2(x−3)1/2=(x−1)2x−1x−3.
- So the integral is ∫(x−1)2x−1x−3dx.
- Let t=x−1x−3=1−x−12. Then dxdt=(x−1)22, i.e. (x−1)2dx=2dt.
- The integral becomes ∫tdt/2=21⋅2t+c=t+c=x−1x−3+c. …
- 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 2024Set eng-2024-05-18-FN1 markMCQQ.∫x55x5+11dx= (A) 5x5+14+c (B) 4x4(x5+1)4/5+c (C) −4x4(x5+1)4/5+c (D) −4x5(x5+1)4/5+c
›Reveal solutionSolution
Rewriting the integrand to expose 1+x−5 as the natural substitution variable solves this cleanly; the answer is −4x4(x5+1)4/5+c.
Concept and Intuition
When an integral mixes a power of x with a root of a polynomial in x, factoring out the highest power of x from inside the root often converts the expression into a function of 1/x (or x−5 here), whose derivative is already present elsewhere in the integrand — a clean substitution.
Step-by-Step Solution
- (x5+1)−1/5=(x5(1+x−5))−1/5=x−1(1+x−5)−1/5.
- So the integrand x−5(x5+1)−1/5=x−6(1+x−5)−1/5.
- Let t=1+x−5, so dt=−5x−6dx⇒x−6dx=−5dt.
- Integral =∫t−1/5(−5dt)=−51⋅4/5t4/5+c=−41t4/5+c. …
- 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 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-2021-08-24-FN1 markMCQQ.If ∫3x{1+3x4}1/7dx=A(1+3x4)B+C, then value of AB= ____ (A) 23 (B) 43 (C) 323 (D) 34
›Reveal solutionSolution
A direct substitution u=1+x4/3 turns the integral into a simple power rule, from which A and B are read off and multiplied.
Concept and Intuition
When the integrand contains x1/3 times a function of x4/3, substituting u=1+x4/3 (whose derivative involves exactly x1/3dx) is the natural simplification.
Step-by-Step Solution
- Let u=1+x4/3. Then du=34x1/3dx⇒x1/3dx=43du.
- The integral ∫x1/3(1+x4/3)1/7dx=43∫u1/7du.
- ∫u1/7du=8/7u8/7=87u8/7.
- So the integral =43⋅87u8/7+C=3221(1+x4/3)8/7+C. …
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