MHT-CET Maths · Differentiation
Logarithmic Differentiation — Logs, Powers, and Long Products
When a function is a product, a quotient, or has a variable in the exponent, take the natural log of both sides FIRST — logs turn products into sums and pull exponents down front, so the differentiation becomes routine.
Why this matters
This is the most mechanical high-yield method in the chapter — 19 PYQs sit here, 9 HARD and 10 MODERATE, yet almost every one follows the SAME three steps. Two shapes dominate the exam: a variable raised to a variable power like (sin x) to the tan x, and a long product (x+1)(2x+1)…(nx+1) whose derivative is asked at x = 0. Master the three steps — take log, differentiate (1/y)·y′, multiply back by y — and most of these become one-minute questions.
Concept 1 of 5
Logarithmic Differentiation — the Method
Intuition
Definition
The three-step method for :
- Take logs: write and simplify using log laws (, ).
- Differentiate both sides: the left becomes (chain rule), the right is now a sum.
- **Multiply back by :** .
The headline result is the variable-base, variable-exponent rule below — both the "treat the exponent as constant" term and the "treat the base as constant" term appear and ADD.
Derivative of f(x) raised to g(x)
- g'(x)\,\log f(x)the term from differentiating the exponent (treat base as constant)
- g(x)\,f'(x)/f(x)the term from differentiating the base (treat exponent as constant)
Worked example
- Take logs: .
- Differentiate both sides; use the product rule on the right: .
- Multiply back by : .
Practice this conceptself-check · 4 quick reps
Try it yourself
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.— does this need logs?
- 2.. Find .
- 3.. State after taking logs.
- 4.Differentiate the exponent of only: .
From the bank · past-year question
[Q148 · 11th May Shift 2 · 2023]
Both terms appear — never use just one
A variable in the exponent kills the power rule
cos⁻¹(sin θ) collapses before you differentiate
Concept 2 of 5
Products, Quotients and Powers via Logs
Intuition
Definition
After , use the three log laws to split:
- Product sum: .
- Quotient difference: .
- Power coefficient: (so a fractional exponent becomes a coefficient ).
Each resulting differentiates to ; then multiply the whole sum by . If the function is already wrapped in a log (), you do NOT have a hidden — just expand the inside with log laws and differentiate the sum directly.
Log of a power-product
Worked example
- Take logs and split: .
- Differentiate term-by-term: .
- Multiply back by : .
Practice this conceptself-check · 4 quick reps
Try it yourself
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.. State .
- 2.Differentiate .
- 3.Coefficient of after of ?
- 4.. Find .
From the bank · past-year question
[Q110 · 3rd May 2nd Shift · 2023]
If y is already a log, there is no 1/y
Simplify before you differentiate —
A fractional exponent becomes a fractional COEFFICIENT
Concept 3 of 5
The Product Chain [(x+1)(2x+1)⋯(nx+1)] Evaluated at x=0
Intuition
Definition
For :
- Take logs: .
- Differentiate: .
- Evaluate at : every so and .
The leftover sum is a standard power-sum (carried in the formula box). If the -th factor is instead, you get . If the product runs and you evaluate at , only the vanishing factor's term survives — handle that by factoring it out, not by the sum.
Power sums (the leftover at x=0)
- pthe outer power (e.g. , , , or ) — it just multiplies the sum
- kthe coefficient of in the -th factor; squared factors give
Worked example
- There is no outer power here (). Take logs: .
- Differentiate: .
- At : every factor is , so and the sum is — note you simply add the coefficients of .
- Therefore .
Practice this conceptself-check · 4 quick reps
Try it yourself
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.at .
- 2.Same product to the power , at .
- 3.with , squared, at .
- 4.at .
From the bank · past-year question
[Q101 · 15th May Shift 1 · 2023]
Substitute x=0 only AFTER differentiating
Squared factor , not
Product like (1-x)(2-x)⋯(n-x) at x=1 — factor, don't sum
The outer power just multiplies the sum
Concept 4 of 5
Change of Base and log-of-a-log Forms
Intuition
Definition
Change of base: (any common base; use natural ). This converts a variable-base logarithm into a QUOTIENT of two natural logs, after which differentiate by the quotient rule. Nested form: ; differentiate this quotient, then evaluate. Many of these are asked at a clean point (, ) where one of the two log terms vanishes, killing half the quotient-rule expression.
Change of base
- \lognatural log (base ) throughout this chapter
- athe base — when it depends on , this is why you must convert
Worked example
- Change base: (since ).
- Write as and differentiate: .
- Tidy: .
Practice this conceptself-check · 4 quick reps
Try it yourself
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Write with natural logs.
- 2.Value of ?
- 3.Simplify denominator.
- 4.
From the bank · past-year question
[Q102 · 11th May Shift 1 · 2024]
Convert the variable base BEFORE differentiating
A vanishing log term kills half the quotient rule
log of a log is NOT (log)²
Concept 5 of 5
Square-Root Quotients with Inverse-Trig Arguments
Intuition
Definition
For where (e.g. ):
- Take logs and use the square root : .
- Differentiate: .
- Multiply back by and evaluate at the given point.
When asked at , note , so , , and at — the whole expression collapses to a single clean number.
Log of a square-root quotient
- uthe inner function, e.g. , with
- 1/2the coefficient produced by the outer square root
Worked example
- Take logs: .
- Differentiate: .
- Multiply back by : .
Practice this conceptself-check · 4 quick reps
Try it yourself
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Value of ?
- 2.Value of at ?
- 3.Coefficient from the outer after taking logs?
- 4.at .
From the bank · past-year question
[Q150 · 11th May Shift 1 · 2024]
Don't forget the chain factor u' on the inverse-trig inner
Compute y at the point — usually y=1 at x=0
Watch which factor is on top — it sets the sign
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (5)
- Logarithmic Differentiation — the Method
Derivative of f(x) raised to g(x)
- Products, Quotients and Powers via Logs
Log of a power-product
- The Product Chain [(x+1)(2x+1)⋯(nx+1)] Evaluated at x=0
Power sums (the leftover at x=0)
- Change of Base and log-of-a-log Forms
Change of base
- Square-Root Quotients with Inverse-Trig Arguments
Log of a square-root quotient
Watch out for (16)
- Both terms appear — never use just one→ Logarithmic Differentiation — the Method
- A variable in the exponent kills the power rule→ Logarithmic Differentiation — the Method
- cos⁻¹(sin θ) collapses before you differentiate→ Logarithmic Differentiation — the Method
- If y is already a log, there is no 1/y→ Products, Quotients and Powers via Logs
- Simplify before you differentiate —→ Products, Quotients and Powers via Logs
- A fractional exponent becomes a fractional COEFFICIENT→ Products, Quotients and Powers via Logs
- Substitute x=0 only AFTER differentiating→ The Product Chain [(x+1)(2x+1)⋯(nx+1)] Evaluated at x=0
- Squared factor , not→ The Product Chain [(x+1)(2x+1)⋯(nx+1)] Evaluated at x=0
- Product like (1-x)(2-x)⋯(n-x) at x=1 — factor, don't sum→ The Product Chain [(x+1)(2x+1)⋯(nx+1)] Evaluated at x=0
- The outer power just multiplies the sum→ The Product Chain [(x+1)(2x+1)⋯(nx+1)] Evaluated at x=0
- Convert the variable base BEFORE differentiating→ Change of Base and log-of-a-log Forms
- A vanishing log term kills half the quotient rule→ Change of Base and log-of-a-log Forms
- log of a log is NOT (log)²→ Change of Base and log-of-a-log Forms
- Don't forget the chain factor u' on the inverse-trig inner→ Square-Root Quotients with Inverse-Trig Arguments
- Compute y at the point — usually y=1 at x=0→ Square-Root Quotients with Inverse-Trig Arguments
- Watch which factor is on top — it sets the sign→ Square-Root Quotients with Inverse-Trig Arguments
Mastery check — 5 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
[Shift || · 2025]
[Q108 · 2nd May Shift 1 · 2023]
[Q131 · 3rd May Shift 2 · 2023]
[Q110 · 2nd May Shift 2 · 2023]
[Q142 · 2nd May Shift 1 · 2023]
Drill every past-year question on this subtopic
19 questions from the bank — paginated, with cart and Word-export support.