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

If yy is a product, a quotient, or — crucially — has a variable in BOTH the base and the exponent, the ordinary power rule and chain rule do not apply directly. Taking the natural log of both sides first turns multiplication into addition and pulls exponents down as multipliers, after which differentiation is straightforward.

Definition

The three-step method for y=f(x)y = f(x):

  • Take logs: write logy=logf(x)\log y = \log f(x) and simplify using log laws (log(ab)=loga+logb\log(ab)=\log a+\log b, log(ap)=ploga\log(a^p)=p\log a).
  • Differentiate both sides: the left becomes 1ydydx\dfrac{1}{y}\dfrac{dy}{dx} (chain rule), the right is now a sum.
  • **Multiply back by yy:** dydx=y[the differentiated right side]\dfrac{dy}{dx} = y\cdot[\text{the differentiated right side}].

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)

ddx[f(x)g(x)]=f(x)g(x)[g(x)logf(x)+g(x)f(x)f(x)]\frac{d}{dx}\left[f(x)^{g(x)}\right] = f(x)^{g(x)}\left[g'(x)\,\log f(x) + g(x)\,\frac{f'(x)}{f(x)}\right]
  • 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

If y=xxy = x^x, find dydx\dfrac{dy}{dx}.
  1. Take logs: logy=xlogx\log y = x\log x.
  2. Differentiate both sides; use the product rule on the right: 1ydydx=1logx+x1x=logx+1\dfrac{1}{y}\dfrac{dy}{dx} = 1\cdot\log x + x\cdot\dfrac{1}{x} = \log x + 1.
  3. Multiply back by y=xxy = x^x: dydx=xx(logx+1)\dfrac{dy}{dx} = x^x(\log x + 1).
Answer:dydx=xx(1+logx)\dfrac{dy}{dx} = x^x(1 + \log x)
Practice this conceptself-check · 4 quick reps

Try it yourself

If y=(cosx)xy = (\cos x)^x, find dydx\dfrac{dy}{dx}.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    y=x2y = x^2 — does this need logs?
  2. 2.
    y=2xy = 2^x. Find dydx\dfrac{dy}{dx}.
  3. 3.
    y=xlogxy = x^{\log x}. State logy\log y after taking logs.
  4. 4.
    Differentiate the exponent of xxx^x only: ddx(xlogx)\dfrac{d}{dx}(x\log x).

From the bank · past-year question

Example 1DifferentiationMODERATE
If y=(sinx)tanxy=(\sin x)^{\tan x}, then dydx\frac{dy}{dx} is equal to

[Q148 · 11th May Shift 2 · 2023]

Both terms appear — never use just one

For fgf^g, the "power rule" alone (gfg1fg f^{g-1}f') and the "exponential rule" alone (fglogfgf^g\log f\cdot g') are each HALF the answer. Logarithmic differentiation produces BOTH terms and adds them. Using only one is the single most common error here.

A variable in the exponent kills the power rule

ddx(xx)\dfrac{d}{dx}(x^x) is NOT xxx1x\cdot x^{x-1}. The power rule ddxxn=nxn1\dfrac{d}{dx}x^n = nx^{n-1} requires nn to be CONSTANT. When the exponent itself depends on xx, take logs.

cos⁻¹(sin θ) collapses before you differentiate

In mixed stems like cos1(sinθ)+xx\cos^{-1}(\sin\theta) + x^x, use cos1(sinθ)=π2θ\cos^{-1}(\sin\theta) = \tfrac{\pi}{2} - \theta (for θ[0,π2]\theta\in[0,\tfrac{\pi}{2}]) to flatten the inverse-trig piece to a simple θ-\theta'; only the xxx^x part needs log differentiation. Mixing the two methods up wastes time.

Concept 2 of 5

Products, Quotients and Powers via Logs

Intuition

When yy is a tangle of products, quotients, and fractional powers, taking logs once flattens the whole thing into a SUM of simple log\log terms. Each term differentiates to (inner)inner\dfrac{(\text{inner})'}{\text{inner}}, and you read off dy/dxdy/dx almost by inspection.

Definition

After logy=log(everything)\log y = \log(\text{everything}), use the three log laws to split:

  • Product \to sum: log(ab)=loga+logb\log(ab) = \log a + \log b.
  • Quotient \to difference: log(a/b)=logalogb\log(a/b) = \log a - \log b.
  • Power \to coefficient: log(ap)=ploga\log(a^p) = p\log a (so a fractional exponent 4/34/3 becomes a coefficient 4/34/3).

Each resulting logu(x)\log u(x) differentiates to u(x)u(x)\dfrac{u'(x)}{u(x)}; then multiply the whole sum by yy. If the function is already wrapped in a log (y=log()y = \log(\cdots)), you do NOT have a hidden 1/y1/y — just expand the inside with log laws and differentiate the sum directly.

Log of a power-product

log ⁣(apbqcr)=ploga+qlogbrlogc\log\!\left(\frac{a^{p}\,b^{q}}{c^{r}}\right) = p\log a + q\log b - r\log c

Worked example

If y=x2x1(x+2)3y = \dfrac{x^2\sqrt{x-1}}{(x+2)^3}, find dydx\dfrac{dy}{dx}.
  1. Take logs and split: logy=2logx+12log(x1)3log(x+2)\log y = 2\log x + \tfrac{1}{2}\log(x-1) - 3\log(x+2).
  2. Differentiate term-by-term: 1ydydx=2x+12(x1)3x+2\dfrac{1}{y}\dfrac{dy}{dx} = \dfrac{2}{x} + \dfrac{1}{2(x-1)} - \dfrac{3}{x+2}.
  3. Multiply back by yy: dydx=x2x1(x+2)3[2x+12(x1)3x+2]\dfrac{dy}{dx} = \dfrac{x^2\sqrt{x-1}}{(x+2)^3}\left[\dfrac{2}{x} + \dfrac{1}{2(x-1)} - \dfrac{3}{x+2}\right].
Answer:dydx=x2x1(x+2)3[2x+12(x1)3x+2]\dfrac{dy}{dx} = \dfrac{x^2\sqrt{x-1}}{(x+2)^3}\left[\dfrac{2}{x} + \dfrac{1}{2(x-1)} - \dfrac{3}{x+2}\right]
Practice this conceptself-check · 4 quick reps

Try it yourself

If y=log ⁣(x3(2x+1)4(x3)5)y = \log\!\left(\dfrac{x^3(2x+1)^4}{(x-3)^5}\right), find dydx\dfrac{dy}{dx}.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    y=x4(x+1)2y = x^4(x+1)^2. State logy\log y.
  2. 2.
    Differentiate log(3x4)\log(3x-4).
  3. 3.
    Coefficient of log(x+5)\log(x+5) after log\log of (x+5)4/3(x+5)^{4/3}?
  4. 4.
    y=log ⁣xx+1y = \log\!\dfrac{x}{x+1}. Find dydx\dfrac{dy}{dx}.

From the bank · past-year question

Example 2DifferentiationMODERATE
If y=loge ⁣(x5(3x4)4/3(x+5)4/3)y=\log_e\!\left(\dfrac{x^{5}\cdot(3x-4)^{4/3}}{(x+5)^{4/3}}\right), then dydx\dfrac{dy}{dx} is equal to

[Q110 · 3rd May 2nd Shift · 2023]

If y is already a log, there is no 1/y

For y=log(expression)y = \log(\text{expression}), expand the inside with log laws and differentiate the SUM directly. The 1ydydx\dfrac{1}{y}\dfrac{dy}{dx} form is only for logy=\log y = \cdots (i.e. you took the log yourself).

Simplify before you differentiate — log1+sinx1sinx\log\sqrt{\frac{1+\sin x}{1-\sin x}}

Some quotient-log stems collapse to a tidy single function. log1+sinx1sinx=logtan ⁣(π4+x2)\log\sqrt{\tfrac{1+\sin x}{1-\sin x}} = \log\tan\!\left(\tfrac{\pi}{4}+\tfrac{x}{2}\right), whose derivative is the clean secx\sec x. Charging in with the quotient rule on the raw fraction works but is far slower and error-prone.

A fractional exponent becomes a fractional COEFFICIENT

log[(x+5)4/3]=43log(x+5)\log[(x+5)^{4/3}] = \tfrac{4}{3}\log(x+5), not 43(x+5)\tfrac{4}{3}(x+5) and not (x+5)4/3log(x+5)^{4/3}\log. Drop the exponent out front; do not leave it on the argument.

Concept 3 of 5

The Product Chain [(x+1)(2x+1)⋯(nx+1)] Evaluated at x=0

Intuition

This is the chapter's signature trick. A long product like [(x+1)(2x+1)(nx+1)]p[(x+1)(2x+1)\cdots(nx+1)]^p looks frightening, but logs turn it into a sum and the magic happens AT x=0x=0: every factor kx+1kx+1 equals 11 there, so y=1y=1 and the messy denominators all collapse, leaving a simple sum like k\sum k or k2\sum k^2.

Definition

For y=[(x+1)(2x+1)(3x+1)(nx+1)]py = [(x+1)(2x+1)(3x+1)\cdots(nx+1)]^{p}:

  • Take logs: logy=pk=1nlog(kx+1)\log y = p\sum_{k=1}^{n}\log(kx+1).
  • Differentiate: 1ydydx=pk=1nkkx+1\dfrac{1}{y}\dfrac{dy}{dx} = p\sum_{k=1}^{n}\dfrac{k}{kx+1}.
  • Evaluate at x=0x=0: every kx+1=1kx+1=1 so y=1y=1 and dydx=pk=1nk\dfrac{dy}{dx} = p\sum_{k=1}^{n}k.

The leftover sum is a standard power-sum (carried in the formula box). If the kk-th factor is k2x+1k^2x+1 instead, you get k2\sum k^2. If the product runs (1x)(2x)(nx)(1-x)(2-x)\cdots(n-x) and you evaluate at x=1x=1, only the vanishing factor's term survives — handle that by factoring it out, not by the sum.

Power sums (the leftover at x=0)

k=1nk=n(n+1)2k=1nk2=n(n+1)(2n+1)6\sum_{k=1}^{n} k = \frac{n(n+1)}{2} \qquad \sum_{k=1}^{n} k^2 = \frac{n(n+1)(2n+1)}{6}
  • pthe outer power (e.g. 22, 44, 3/23/2, or nn) — it just multiplies the sum
  • kthe coefficient of xx in the kk-th factor; squared factors give k2\sum k^2

Worked example

If y=(1+2x)(1+5x)(1+7x)y = (1+2x)(1+5x)(1+7x), find dydx\dfrac{dy}{dx} at x=0x=0.
  1. There is no outer power here (p=1p=1). Take logs: logy=log(1+2x)+log(1+5x)+log(1+7x)\log y = \log(1+2x) + \log(1+5x) + \log(1+7x).
  2. Differentiate: 1ydydx=21+2x+51+5x+71+7x\dfrac{1}{y}\dfrac{dy}{dx} = \dfrac{2}{1+2x} + \dfrac{5}{1+5x} + \dfrac{7}{1+7x}.
  3. At x=0x=0: every factor is 11, so y=1y=1 and the sum is 2+5+7=142+5+7=14 — note you simply add the coefficients of xx.
  4. Therefore dydx0=14\dfrac{dy}{dx}\Big|_{0} = 14.
Answer:dydxx=0=14\dfrac{dy}{dx}\Big|_{x=0} = 14
Practice this conceptself-check · 4 quick reps

Try it yourself

If y=(1+x)(1+4x)(1+9x)(1+16x)y = (1+x)(1+4x)(1+9x)(1+16x), find dydx\dfrac{dy}{dx} at x=0x=0.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    y=[(x+1)(2x+1)(nx+1)]2y=[(x+1)(2x+1)\cdots(nx+1)]^2 at x=0x=0.
  2. 2.
    Same product to the power 44, at x=0x=0.
  3. 3.
    y=(x+1)(4x+1)(9x+1)y=(x+1)(4x+1)(9x+1)\cdots with (k2x+1)(k^2x+1), squared, at x=0x=0.
  4. 4.
    y=(1+x)(1+x2)(1+x4)y=(1+x)(1+x^2)(1+x^4)\cdots at x=0x=0.

From the bank · past-year question

Example 3DifferentiationMODERATE
If y=[(x+1)(2x+1)(3x+1)(nx+1)]2y = [(x+1)(2x+1)(3x+1)\cdots(nx+1)]^{2}, then dydx\frac{dy}{dx} at x=0x=0 is

[Q101 · 15th May Shift 1 · 2023]

Substitute x=0 only AFTER differentiating

If you plug x=0x=0 into yy before differentiating, you get the constant 11 and derivative 00 — wrong. Differentiate the log-sum fully (keep xx), THEN set x=0x=0 so the denominators collapse to 11.

Squared factor \Rightarrow k2\sum k^2, not k\sum k

If the kk-th factor is k2x+1k^2x+1 (i.e. 1,4x+1,9x+1,1, 4x+1, 9x+1,\ldots), differentiating log(k2x+1)\log(k^2x+1) gives k2k2x+1\dfrac{k^2}{k^2x+1}, so the leftover sum is k2=n(n+1)(2n+1)6\sum k^2 = \tfrac{n(n+1)(2n+1)}{6}. Don't reflexively write k\sum k.

Product like (1-x)(2-x)⋯(n-x) at x=1 — factor, don't sum

When a factor itself VANISHES at the evaluation point (here (1x)=0(1-x)=0 at x=1x=1), the 1yy\dfrac{1}{y}y' form blows up. Instead write y=(1x)g(x)y=(1-x)\,g(x); then y(1)=1g(1)y'(1) = -1\cdot g(1), where g(1)g(1) is the product of the remaining factors at x=1x=1.

The outer power just multiplies the sum

A power 3/23/2 or nn on the whole product becomes a coefficient pp in logy=plog()\log y = p\sum\log(\cdots). So the answer is always pkp\sum k (or pk2p\sum k^2) — e.g. power 3/23/2 gives 32n(n+1)2=3n(n+1)4\tfrac{3}{2}\cdot\tfrac{n(n+1)}{2} = \tfrac{3n(n+1)}{4}.

Concept 4 of 5

Change of Base and log-of-a-log Forms

Intuition

A logarithm with a VARIABLE base — like logsinx(tanx)\log_{\sin x}(\tan x) — can't be differentiated as is. Change it to natural logs first via logab=logbloga\log_a b = \dfrac{\log b}{\log a}; now both top and bottom are ordinary logs and the quotient rule finishes the job.

Definition

Change of base: logab=logbloga\log_a b = \dfrac{\log b}{\log a} (any common base; use natural log\log). This converts a variable-base logarithm into a QUOTIENT of two natural logs, after which differentiate by the quotient rule. Nested form: logx2(logx)=log(logx)log(x2)=log(logx)2logx\log_{x^2}(\log x) = \dfrac{\log(\log x)}{\log(x^2)} = \dfrac{\log(\log x)}{2\log x}; differentiate this quotient, then evaluate. Many of these are asked at a clean point (x=π4x = \tfrac{\pi}{4}, x=ex = e) where one of the two log terms vanishes, killing half the quotient-rule expression.

Change of base

logab=logbloga\log_a b = \frac{\log b}{\log a}
  • \lognatural log (base ee) throughout this chapter
  • athe base — when it depends on xx, this is why you must convert

Worked example

If y=logx(e)y = \log_{x}(e), find dydx\dfrac{dy}{dx} for x>0, x1x>0,\ x\neq 1.
  1. Change base: y=logelogx=1logxy = \dfrac{\log e}{\log x} = \dfrac{1}{\log x} (since loge=1\log e = 1).
  2. Write as (logx)1(\log x)^{-1} and differentiate: dydx=1(logx)21x\dfrac{dy}{dx} = -1\cdot(\log x)^{-2}\cdot\dfrac{1}{x}.
  3. Tidy: dydx=1x(logx)2\dfrac{dy}{dx} = -\dfrac{1}{x(\log x)^2}.
Answer:dydx=1x(logx)2\dfrac{dy}{dx} = -\dfrac{1}{x(\log x)^2}
Practice this conceptself-check · 4 quick reps

Try it yourself

If f(x)=logx2(logx)f(x) = \log_{x^2}(\log x), find f(x)f'(x) at x=ex=e.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Write logsinx(tanx)\log_{\sin x}(\tan x) with natural logs.
  2. 2.
    Value of logtan(π/4)\log\tan(\pi/4)?
  3. 3.
    Simplify logx2(logx)\log_{x^2}(\log x) denominator.
  4. 4.
    logaa=?\log_a a = ?

From the bank · past-year question

Example 4DifferentiationHARD
If y=logsinx(tanx)y = \log_{\sin x}(\tan x), then dydxx=π/4\left.\frac{dy}{dx}\right|_{x=\pi/4} has the value

[Q102 · 11th May Shift 1 · 2024]

Convert the variable base BEFORE differentiating

ddxlogsinx(tanx)\dfrac{d}{dx}\log_{\sin x}(\tan x) cannot be done with the 1u\dfrac{1}{u} rule directly — the base sinx\sin x is not constant. Always rewrite as logtanxlogsinx\dfrac{\log\tan x}{\log\sin x} first, then use the quotient rule.

A vanishing log term kills half the quotient rule

At nice points (x=π/4x=\pi/4 gives logtanx=0\log\tan x = 0; x=ex=e gives loglogx=0\log\log x = 0), the quotient-rule term multiplying that zero disappears. Spot the vanishing term FIRST — it saves most of the algebra. The bank answer for logsinxtanx\log_{\sin x}\tan x at π/4\pi/4 is 4log2-4\log 2.

log of a log is NOT (log)²

log(logx)\log(\log x) is a composition (log applied to logx\log x), with derivative 1logx1x\dfrac{1}{\log x}\cdot\dfrac{1}{x}. It is not (logx)2(\log x)^2, whose derivative would be 2logx1x2\log x\cdot\tfrac{1}{x}. Keep the two straight.

Concept 5 of 5

Square-Root Quotients with Inverse-Trig Arguments

Intuition

A function like 1sin1x1+sin1x\sqrt{\dfrac{1-\sin^{-1}x}{1+\sin^{-1}x}} bundles a square root, a quotient, and an inverse-trig term. Logs flatten all three at once: the  \sqrt{\ } becomes a 12\tfrac{1}{2} coefficient and the quotient becomes a difference, leaving a short sum to differentiate and evaluate.

Definition

For y=1u1+uy = \sqrt{\dfrac{1 - u}{1 + u}} where u=u(x)u = u(x) (e.g. u=sin1xu = \sin^{-1}x):

  • Take logs and use the square root \to 12\tfrac{1}{2}: logy=12[log(1u)log(1+u)]\log y = \tfrac{1}{2}[\log(1-u) - \log(1+u)].
  • Differentiate: 1ydydx=12[u1uu1+u]\dfrac{1}{y}\dfrac{dy}{dx} = \tfrac{1}{2}\left[\dfrac{-u'}{1-u} - \dfrac{u'}{1+u}\right].
  • Multiply back by yy and evaluate at the given point.

When asked at x=0x=0, note sin10=0\sin^{-1}0 = 0, so u=0u=0, y=1y=1, and u=11x2=1u' = \dfrac{1}{\sqrt{1-x^2}} = 1 at x=0x=0 — the whole expression collapses to a single clean number.

Log of a square-root quotient

log1u1+u=12[log(1u)log(1+u)]\log\sqrt{\frac{1-u}{1+u}} = \frac{1}{2}\Big[\log(1-u) - \log(1+u)\Big]
  • uthe inner function, e.g. sin1x\sin^{-1}x, with u=1/1x2u' = 1/\sqrt{1-x^2}
  • 1/2the coefficient produced by the outer square root

Worked example

If y=1x1+xy = \sqrt{\dfrac{1-x}{1+x}}, find dydx\dfrac{dy}{dx}.
  1. Take logs: logy=12[log(1x)log(1+x)]\log y = \tfrac{1}{2}[\log(1-x) - \log(1+x)].
  2. Differentiate: 1ydydx=12[11x11+x]=12(1+x)(1x)(1x)(1+x)=11x2\dfrac{1}{y}\dfrac{dy}{dx} = \tfrac{1}{2}\left[\dfrac{-1}{1-x} - \dfrac{1}{1+x}\right] = \tfrac{1}{2}\cdot\dfrac{-(1+x)-(1-x)}{(1-x)(1+x)} = \dfrac{-1}{1-x^2}.
  3. Multiply back by y=1x1+xy = \sqrt{\tfrac{1-x}{1+x}}: dydx=11x21x1+x\dfrac{dy}{dx} = -\dfrac{1}{1-x^2}\sqrt{\dfrac{1-x}{1+x}}.
Answer:dydx=11x21x1+x\dfrac{dy}{dx} = -\dfrac{1}{1-x^2}\sqrt{\dfrac{1-x}{1+x}}
Practice this conceptself-check · 4 quick reps

Try it yourself

If y=1logx1+logxy = \sqrt{\dfrac{1-\log x}{1+\log x}}, find dydx\dfrac{dy}{dx} at x=1x=1.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Value of sin10\sin^{-1}0?
  2. 2.
    Value of ddxsin1x\dfrac{d}{dx}\sin^{-1}x at x=0x=0?
  3. 3.
    Coefficient from the outer  \sqrt{\ } after taking logs?
  4. 4.
    y=1sin1x1+sin1xy=\sqrt{\tfrac{1-\sin^{-1}x}{1+\sin^{-1}x}} at x=0x=0.

From the bank · past-year question

Example 5DifferentiationMODERATE
If y=1sin1(x)1+sin1(x)y = \sqrt{\frac{1-\sin^{-1}(x)}{1+\sin^{-1}(x)}}, then dydx\frac{dy}{dx} at x=0x=0 and y=1y=1 is

[Q150 · 11th May Shift 1 · 2024]

Don't forget the chain factor u' on the inverse-trig inner

When u=sin1xu = \sin^{-1}x, each log(1±u)\log(1\pm u) differentiates to ±u1±u\dfrac{\pm u'}{1\pm u} with u=11x2u' = \dfrac{1}{\sqrt{1-x^2}}. Dropping the uu' (treating uu as xx) is a frequent error; at x=0x=0 it happens to equal 11, but you must include it in general.

Compute y at the point — usually y=1 at x=0

You multiply 1yy\dfrac{1}{y}y' by yy to finish. At x=0x=0 the quotient under the root is 11=1\tfrac{1}{1}=1, so y=1y=1 and the multiply-back is trivial. Forgetting to put yy back (leaving only 1yy\tfrac{1}{y}y') gives the wrong magnitude when y1y\neq 1.

Watch which factor is on top — it sets the sign

1sin1x1+sin1x\sqrt{\tfrac{1-\sin^{-1}x}{1+\sin^{-1}x}} gives 1-1 at x=0x=0; flipping to 1+sin1x1sin1x\sqrt{\tfrac{1+\sin^{-1}x}{1-\sin^{-1}x}} gives +1+1. The numerator/denominator order flips every sign — read the stem carefully before reaching for a memorised answer.

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)

    ddx[f(x)g(x)]=f(x)g(x)[g(x)logf(x)+g(x)f(x)f(x)]\frac{d}{dx}\left[f(x)^{g(x)}\right] = f(x)^{g(x)}\left[g'(x)\,\log f(x) + g(x)\,\frac{f'(x)}{f(x)}\right]
  • Products, Quotients and Powers via Logs

    Log of a power-product

    log ⁣(apbqcr)=ploga+qlogbrlogc\log\!\left(\frac{a^{p}\,b^{q}}{c^{r}}\right) = p\log a + q\log b - r\log c
  • The Product Chain [(x+1)(2x+1)⋯(nx+1)] Evaluated at x=0

    Power sums (the leftover at x=0)

    k=1nk=n(n+1)2k=1nk2=n(n+1)(2n+1)6\sum_{k=1}^{n} k = \frac{n(n+1)}{2} \qquad \sum_{k=1}^{n} k^2 = \frac{n(n+1)(2n+1)}{6}
  • Change of Base and log-of-a-log Forms

    Change of base

    logab=logbloga\log_a b = \frac{\log b}{\log a}
  • Square-Root Quotients with Inverse-Trig Arguments

    Log of a square-root quotient

    log1u1+u=12[log(1u)log(1+u)]\log\sqrt{\frac{1-u}{1+u}} = \frac{1}{2}\Big[\log(1-u) - \log(1+u)\Big]

Watch out for (16)

Mastery check — 5 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1DifferentiationHARD
The first derivative of the function (cos1(sin1+x2)+xx)\left(\cos^{-1}\left(\sin\sqrt{\frac{1+x}{2}}\right) + x^x\right) with respect to x at x=1x = 1 is

[Shift || · 2025]

Example 2DifferentiationMODERATE
If y=e4x(x4)3/4(x+3)3/4y = e^{4x} \cdot \dfrac{(x-4)^{3/4}}{(x+3)^{3/4}} then dydx=\dfrac{dy}{dx} =

[Q108 · 2nd May Shift 1 · 2023]

Example 3DifferentiationMODERATE
If y=[(x+1)(2x+1)(3x+1)(nx+1)]4y=[(x+1)(2x+1)(3x+1)\cdots(nx+1)]^{4} then dydx\frac{dy}{dx} at x=0x=0 is

[Q131 · 3rd May Shift 2 · 2023]

Example 4DifferentiationHARD
If f(x)=logx2(logex)f(x) = \log_{x^2}(\log_e x), then f(x)f'(x) at x=ex = e is

[Q110 · 2nd May Shift 2 · 2023]

Example 5DifferentiationHARD
If y=1sin1x1+sin1xy = \sqrt{\dfrac{1 - \sin^{-1}x}{1 + \sin^{-1}x}}, then dydx\dfrac{dy}{dx} at x=0x = 0 is

[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.