Q.∫2−3xdx=
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Natural Logarithm Integration
Natural Logarithm Integration: From Intuition to Formula
You already know that integration is the reverse of differentiation. So the first question is: what function, when differentiated, gives x1?
You know dxd(xn)=nxn−1. Trying to find a function whose derivative is x−1, the power rule would give 0x0, which is undefined. That's the clue — x1 doesn't fit the power rule pattern.
The function that fills this gap is the natural logarithm, logx. Its derivative is exactly x1 (for x>0), so integration reverses this:
∫x1dx=log∣x∣+C
The absolute value ∣x∣ is crucial — it extends the formula to negative x, because logx is only defined for positive numbers, but x1 is defined for all x=0.
Why the absolute value?
For x>0, differentiating log∣x∣ gives x1. For x<0, log∣x∣=log(−x), and its derivative is −x1⋅(−1)=x1. Same result, so log∣x∣ works for both sides.
The generalised form
The real power comes when the numerator is the derivative of the denominator:
∫f(x)f′(x)dx=log∣f(x)∣+C
This is the logarithmic integration pattern.
Example to see it in action
Find ∫x2+12xdx. Here f(x)=x2+1, so f′(x)=2x — the numerator matches. Therefore:
∫x2+12xdx=log∣x2+1∣+C=log(x2+1)+C
(dropping the absolute value since x2+1 is always positive).
What if the numerator doesn't match exactly?
For ∫x2+1xdx, the derivative of the denominator is 2x but you only have x. Adjust by factoring:
∫x2+1xdx=21∫x2+12xdx=21log∣x2+1∣+C
When the numerator is a constant multiple of the derivative of the denominator, factor out that constant: ∫f(x)k⋅f′(x)dx=klog∣f(x)∣+C.
Common mistake to avoid …
∫a+bxdx=b1log∣a+bx∣+c; here b=−3. …
∫a+bxdx=b1log∣a+bx∣+c; here b=−3.
Let u=2−3x, so du=−3dx, i.e. dx=−31du. Then …
Showing the 12 most recent of 25 on this concept.
- CBSE 2026Set A1 markMCQQ.∫x−1dx=(a) log∣x+1∣+k(b) −log∣x+1∣+k(c) log∣x−1∣+k(d) logx+k
›Reveal solutionSolution
Standard log integral: ∫x−1dx=log∣x−1∣+k.
…
- CBSE 2026Set ANNUAL1 markMCQQ.∫ tan x dx equals(a) log|sin x| + C(b) log|cos x| + C(c) log|sec x| + C(d) log|cot x| + C
›Reveal solutionSolution
Write tanx as sinx/cosx and integrate by substitution u=cosx.
∫tanxdx=∫cosxsinxdx
Let u=cosx, so du=−sinxdx, i.e. sinxdx=−du.
…
- CBSE 2025Set E1 markMCQQ.∫xlogxdx=(a) logx+k(b) (logx)2+k(c) log(logx)+k(d) logx1+k
›Reveal solutionSolution
With t=logx, the integral becomes ∫tdt=log(logx)+k.
Let t=logx, so dt=x1dx. Then …
- CBSE 2025Set E1 markMCQQ.∫x2−9x−3dx=(a) log(x−3)+k(b) log(x+3)+k(c) −(x+3)21+k(d) 2x2−3x+k
›Reveal solutionSolution
x2−9x−3=x+31, so the integral is log(x+3)+k.
Factor the denominator using x2−9=(x−3)(x+3):
x2−9x−3=(x−3)(x+3)x−3=x+31. …
- CBSE 2024Set D1 markMCQQ.∫secxdx=(a) log∣secx+tanx∣+c(b) log∣secx−tanx∣+c(c) logsecx+c(d) tan5x+c
›Reveal solutionSolution
∫secxdx=log∣secx+tanx∣+c.
Multiplying numerator and denominator by (secx+tanx) and substituting t=secx+tanx gives the standard result:
…
- CBSE 2024Set D1 markMCQQ.∫2−3xdx=(a) −3log∣2−3x∣+c(b) −31log∣2−3x∣+c(c) −log∣2−3x∣+c(d) 2tan−1x4+c
›Reveal solutionSolution
∫a+bxdx=b1log∣a+bx∣+c; here b=−3.
Let u=2−3x, so du=−3dx, i.e. dx=−31du. Then …
- CBSE 2024Set ANNUAL1 markQ.Evaluate ∫1+cosxsinxdx.
›Reveal solutionSolution
Substitute u=1+cosx; then du=−sinxdx, turning the integral into a simple udu form.
∫1+cosxsinxdx
Let u=1+cosx, so du=−sinxdx, i.e. sinxdx=−du.
…
- CBSE 2024Set ANNUAL1 markMCQQ.∫23x1dx=(a) 32logx(b) log(23)(c) log(32)(d) none of these
›Reveal solutionSolution
Integrate 1/x to get log x, then apply the limits and combine using log rules.
…
- CBSE 2024Set ANNUAL1 markQ.Evaluate : ∫3x+2dx.
›Reveal solutionSolution
Direct substitution u=3x+2.
∫3x+2dx
Let u=3x+2⟹du=3dx⟹dx=3du. …
- CBSE 2023Set E1 markMCQQ.∫x2−4x+2dx=(a) log∣x+2∣+k(b) log∣x2−4∣+k(c) log∣x−2∣+k(d) logx−2x+2+k
›Reveal solutionSolution
Cancelling (x+2) reduces the integrand to x−21, whose integral is log∣x−2∣+k.
Factor the denominator: x2−4=(x−2)(x+2).
…
- CBSE 2023Set E1 markMCQQ.∫2cosx+3sinx3cosx−2sinxdx=(a) 2cosx+3sinx+k(b) log∣2cosx+3sinx∣+k(c) tan−1(3sin2x)+k(d) 2tan2x+k
›Reveal solutionSolution
Since dxd(2cosx+3sinx)=3cosx−2sinx, the integral is of the form ∫ff′=log∣f∣.
Let f(x)=2cosx+3sinx. Then f′(x)=−2sinx+3cosx=3cosx−2sinx, which is the numerator.
Using ∫f(x)f′(x)dx=log∣f(x)∣+k: …
- CBSE 2023Set E1 markMCQQ.∫x3+2x3x2+2dx=(a) sin−1(x3+2x)+k(b) tan−1(3x2+2)+k(c) log∣3x2+2∣+k(d) log∣x3+2x∣+k
›Reveal solutionSolution
Since dxd(x3+2x)=3x2+2, the integral is ∫ff′=log∣f∣.
Let f(x)=x3+2x. Then f′(x)=3x2+2, exactly the numerator.
Using ∫f(x)f′(x)dx=log∣f(x)∣+k: …
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