Q.Integrate the function: ∫x1/2+x1/31dx
[Hint: put x=t6]
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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 hint x=t6 eliminates fractional exponents by making the denominator a polynomial in t.
Step 1: Substitute x=t6, so dx=6t5dt. Then
x1/2=t3 and x1/3=t2.
Step 2: The integral becomes
∫t3+t26t5dt=∫t2(t+1)6t5dt=6∫t+1t3dt.
Step 3: Perform polynomial division: t+1t3=t2−t+1−t+11.
Thus …
The key idea is to eliminate fractional exponents by substituting x=t6, which turns the integrand into a rational function. After simplifying and performing polynomial division, the integral evaluates to 2x−33x+66x−6log(1+6x)+C.
Why this substitution works
When you see fractional powers like x1/2 and x1/3, the exponents have denominators 2 and 3. The least common multiple of 2 and 3 is 6. So if we set x=t6, then:
- x1/2=(t6)1/2=t3
- x1/3=(t6)1/3=t2
Both become simple integer powers of t. The hint in the problem is exactly this — it’s the cleanest way to handle mixed fractional exponents.
-
Perform the substitution
Let x=t6. Then dx=6t5dt. The integral becomes:
∫x1/2+x1/31dx=∫t3+t21⋅6t5dt
-
Simplify the integrand
Factor the denominator: t3+t2=t2(t+1). So:
∫t2(t+1)6t5dt=∫t+16t3dt
The t2 cancels, leaving a much simpler rational function.
-
Perform polynomial division
The numerator t3 has a higher degree than the denominator t+1, so we divide:
t+1t3=t2−t+1−t+11
Let’s verify: (t+1)(t2−t+1)=t3−t2+t+t2−t+1=t3+1. So indeed t3=(t+1)(t2−t+1)−1, giving the result above.
Therefore:
∫t+16t3dt=6∫(t2−t+1−t+11)dt
- Integrate term by term
=6(3t3−2t2+t−log∣t+1∣)+C
Simplify the coefficients:
=2t3−3t2+6t−6log∣t+1∣+C
-
Substitute back to x
Since t=x1/6, we have:
- t3=(x1/6)3=x1/2=x …
Method: LCM substitution x=tk to clear fractional exponents
Use this whenever the integrand mixes roots like x1/2 and x1/3: substitute x=tk with k=lcm of the denominators to turn everything into a rational function of t.
Steps
Step 1: Choose the exponent.
Take k=lcm(m,n) of the fractional-power denominators (here lcm(2,3)=6), so every root becomes an integer power of t. Remember dx=ktk−1dt.
Step 2: Simplify to a rational function. …
Common Mistakes
Mistake 1: Forgetting dx=6t5dt.
Why it's wrong: with x=t6 the differential is not dt; omitting the 6t5 changes the whole integrand. Correct approach: always transform dx under a substitution.
Mistake 2: Not choosing the LCM exponent.
Why it's wrong: using x=t2 or x=t3 leaves one root fractional; only x=tlcm(2,3)=t6 clears both. Correct approach: take the LCM of the denominators. …
Showing the 12 most recent of 51 on this concept.
- AP EAPCET 2023Set eng-2023-05-17-FN1 markMCQQ.∫(x3m+x2m+xm)(2x2m+3xm+6)m1dx= (A) 6(m+1)1(2x3m+3x2m+6xm)mm+1+C (B) 6(m+1)1(2x3m+3x2m+6xm)mm−1+C (C) 6(m+1)1(2x3m+3x2m+6)mm+1+C (D) 6(m−1)1(2x3m+mx2m+6xm)mm−1+C
›Reveal solutionSolution
Recognising that 2x3m+3x2m+6xm equals xm times the bracket under the 1/m-power root lets the whole integrand be rewritten as (a constant times) g1/mg′ for g=2x3m+3x2m+6xm — a pure "power rule" integral.
Concept and Intuition
Whenever an integrand looks like g(x)1/m⋅g′(x) (up to a constant factor), the antiderivative is immediately 1/m+1g1/m+1. The main work here is algebraic: spotting that the "outside" factor (x3m+x2m+xm) is secretly related to the derivative of g=2x3m+3x2m+6xm, and that the "inside" bracket (2x2m+3xm+6) is just g/xm.
Step-by-Step Solution
- Let g=2x3m+3x2m+6xm. Factor: g=xm(2x2m+3xm+6), so 2x2m+3xm+6=g/xm, and (2x2m+3xm+6)1/m=g1/m/x (since (g/xm)1/m=g1/m/x).
- Differentiate g: g′=6mx3m−1+6mx2m−1+6mxm−1=6mxm−1(x2m+xm+1).
- Rewrite the integrand: (x3m+x2m+xm)(2x2m+3xm+6)1/m=xm(x2m+xm+1)⋅xg1/m=xm−1(x2m+xm+1)g1/m.
- From step 2, xm−1(x2m+xm+1)=6mg′. So the integrand equals 6mg′g1/m. …
- 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 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 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-2022-07-07-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 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 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 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 2026Set eng-2026-05-12-AN1 markMCQQ.If ∫esin3x(sin8x+2sin5x)cosxdx=31esin3xf(x)+c, then f(x)= (A) sin8x (B) sin6x (C) cos8x (D) cos6x
›Reveal solutionSolution
Substituting t=sinx and testing f(t)=t6 against the target derivative confirms f(x)=sin6x.
Concept and Intuition
When an integral has the form eg(x)⋅(stuff)⋅g′(x), substituting u=g(x) turns it into ∫eu⋅h(u)du for a polynomial h. Matching the answer's assumed shape 31euf and differentiating (product rule, since both eu and f depend on u) lets us solve for f by comparing polynomial coefficients — much safer than guessing an antiderivative by inspection alone.
Step-by-Step Solution
- Let t=sinx, so dt=cosxdx. The integral becomes ∫et3(t8+2t5)dt.
- We're told this equals 31et3f(t)+c. Differentiate the RHS w.r.t. t: dtd[31et3f(t)]=31[3t2et3f(t)+et3f′(t)]=et3[t2f(t)+31f′(t)].
- This must equal et3(t8+2t5), so t2f(t)+31f′(t)=t8+2t5. …
- AP EAPCET 2024Set eng-2024-05-18-FN1 markMCQQ.∫(tan7x+tanx)dx= (A) 12tan2x(2tan4x−3tan2x+6)+c (B) 6tan2x−4tan5x+2tan4x+c (C) 6tan2x(tan4x+3tan2x+4)+c (D) 12tanx(tan4x−3tan2x+6)+c
›Reveal solutionSolution
Factoring tan7x+tanx using the sum-of-like-terms identity t7+t=t(t6+1)=t(t2+1)(t4−t2+1) exposes the sec2x needed for a clean t=tanx substitution. Answer: 12tan2x(2tan4x−3tan2x+6)+c.
Concept and Intuition
Whenever an integrand is built purely from powers of tanx together with an explicit or hidden sec2x, substituting t=tanx turns it into a polynomial integral — the key is recognizing that t6+1=(t2+1)(t4−t2+1) supplies exactly the sec2x=1+tan2x factor needed.
Step-by-Step Solution
- tan7x+tanx=tanx(tan6x+1)=tanx(tan2x+1)(tan4x−tan2x+1)=tanxsec2x(tan4x−tan2x+1).
- Let t=tanx, dt=sec2xdx. Integral becomes ∫t(t4−t2+1)dt=∫(t5−t3+t)dt.
- =6t6−4t4+2t2+c=6tan6x−4tan4x+2tan2x+c. …
- 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 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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