Q.Differentiate w.r.t. x: log[log(logx5)].
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Chain Rule
The Chain Rule: Why It Makes Sense
Imagine you're assembling a toy. First you put part A into part B, then you put that combined piece into part C. The final toy's position depends on how you moved A, which then affected B, which then affected C. That's exactly what the chain rule captures — how a change in the first variable ripples through a sequence of functions to affect the final output.
Let's make this concrete. Suppose you have a function f that depends on g, and g itself depends on x:
y=f(g(x))
You want to know: if x changes by a tiny amount, how much does y change? The answer isn't just f′(g(x)) — because g(x) itself changes when x changes. You have to multiply the two rates:
- How fast does g change with respect to x? That's g′(x).
- How fast does f change with respect to its input g? That's f′(g(x)).
The total effect is the product:
dxdy=f′(g(x))⋅g′(x)
In Leibniz notation, this looks even more natural: dxdy=dudy⋅dxdu, where u=g(x). The du's "cancel" like fractions — though this is just a helpful memory aid, not a rigorous proof.
The Precise Statement
Chain Rule (single variable): If g is differentiable at x and f is differentiable at g(x), then the composite function h(x)=f(g(x)) is differentiable at x, and
h′(x)=f′(g(x))⋅g′(x)
That's it. One multiplication. But the power is enormous — it lets you differentiate almost any nested function.
A Simple Example
Differentiate h(x)=sin(3x2).
Here f(u)=sinu and g(x)=3x2. Then:
- f′(u)=cosu, so f′(g(x))=cos(3x2)
- g′(x)=6x
Multiply: h′(x)=cos(3x2)⋅6x=6xcos(3x2)
The most common mistake is forgetting to multiply by the inner derivative. Students often write dxdsin(3x2)=cos(3x2) and stop — that's wrong. The chain rule demands you also multiply by 6x.
Why It's Called a "Chain"
Think of a chain of links: x→g→f. Each link has its own rate of change. To find the total rate from x to f, you multiply the rates of each link. If you had three functions — say h(x)=f(g(k(x))) — you'd multiply three derivatives: …
Concept: Chain Rule – differentiate the outermost function, then multiply by the derivative of the inner function, repeating until the innermost is reached.
Step 1: Let y=log[log(logx5)]. The outermost function is log(⋅), so
dxdy=log(logx5)1⋅dxd[log(logx5)].
Step 2: Differentiate log(logx5):
dxd[log(logx5)]=logx51⋅dxd(logx5). …
The derivative of log[log(logx5)] is found by applying the chain rule three times in succession. The final result is xlogx5⋅log(logx5)5.
The key idea here is the chain rule — when you have a function nested inside another function, you differentiate from the outermost layer inward, multiplying the derivatives at each step. This problem has three layers of nesting: the outer log, then another log, then log(x5), and finally x5 itself. Each layer is a function of the one below it.
Think of it like peeling an onion: start with the outermost peel (the first log), differentiate it, then move to the next layer, and so on, until you reach the innermost x5. At each step, you leave the inner part untouched while differentiating the outer part.
Let’s work through it step by step.
-
Identify the structure.
Let y=log[log(logx5)]. Here, log means the natural logarithm (base e), as is standard in calculus. The innermost part is x5, then log(x5), then log(logx5), and finally the outermost log.
-
Apply the chain rule — first layer.
The derivative of log(u) with respect to u is u1. So, treating u=log(logx5), we have:
dxdy=log(logx5)1⋅dxd[log(logx5)].
- Second layer — differentiate log(logx5). Now let v=logx5. Then dxd[logv]=v1⋅dxdv. So:
dxd[log(logx5)]=logx51⋅dxd(logx5).
- Third layer — differentiate log(x5). Here, log(x5)=5logx (using the logarithm power rule). Its derivative is:
dxd(logx5)=x5.
Alternatively, you can use the chain rule directly: derivative of log(x5) is x51⋅5x4=x5. Same result.
- Combine all layers. Multiply everything together: …
Method: Chain Rule for Multiply-Nested Logarithms — Peel from Outside In
Use this method whenever a function is built from several logarithms nested inside one another, such as log[log(logx5)].
Steps
Step 1: List the layers from outermost to innermost
Identify each nested function separately before differentiating anything. Here: outermost is log(⋅), next is log(⋅) again (applied to logx5), and innermost is log(x5), which itself depends on x5.
Step 2: Differentiate the outermost layer, leaving the inside untouched
dxdlogu=u1⋅dxdu,u=log(logx5)
This produces the factor log(logx5)1, multiplied by the derivative of the next layer in.
Step 3: Repeat for each successive layer
Differentiate log(logx5) the same way, treating v=logx5 as its inner function, producing another factor logx51 multiplied by dxd(logx5).
Step 4: Differentiate the innermost layer using the logarithm power rule …
Common Mistakes
Mistake 1: Stopping after differentiating only one or two of the nested logs
Why it's wrong: with three layers of nesting, skipping a layer (for example differentiating the outer log and jumping straight to x5 without the middle logx51 factor) drops an entire reciprocal factor from the answer. Correct approach: explicitly list every layer before starting, and confirm the final answer has exactly one factor per layer.
Mistake 2: Confusing log(x5) with (logx)5 or log2(x5)
Why it's wrong: the brackets in log[log(logx5)] specify that each log applies to the entire result of the previous step, not that anything is being raised to a power; misreading the nesting leads to differentiating a completely different function. Correct approach: carefully identify what each logarithm's argument actually is, from the innermost bracket outward, before writing any derivative. …
Showing the 12 most recent of 23 on this concept.
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.If f(x)=22xlog(3x−2), then f′(2)= (A) log44log2log4+3 (B) 2log48(log2)2+3 (C) 2log48(log4)2+3 (D) 2log48log2log4+3
›Reveal solutionSolution
Differentiate f(x)=22xlog(3x−2) using the chain rule on the square root and the product rule inside; at x=2 this evaluates to 2log48log2log4+3.
Concept and Intuition
Whenever a function is a square root of a product, write f=L so f′=2LL′ (chain rule), then find L′ using the product rule since L(x)=22x⋅log(3x−2) is a product of an exponential and a log term. This two-layer differentiation is the key technique.
Step-by-Step Solution
- Let L(x)=22xlog(3x−2), so f(x)=L(x) and f′(x)=2L(x)L′(x).
- Evaluate L(2): 22(2)=24=16; log(3(2)−2)=log4. So L(2)=16log4, and L(2)=16log4=4log4.
- Find L′(x) by the product rule: L′(x)=dxd[22x]log(3x−2)+22x⋅dxd[log(3x−2)].
- dxd22x=22x⋅ln2⋅2=2⋅22xlog2 (using log as natural log consistently), i.e. 22x+1log2.
- dxdlog(3x−2)=3x−23.
- So L′(x)=22x+1log2⋅log(3x−2)+22x⋅3x−23. …
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.If y=coshx+coshx, then dxdy= (A) 4y(y2+coshx)sinhx(2y2+2coshx+1) (B) 4y(y2−coshx)sinhx(2y2−2coshx−1) (C) 4ycoshxsinhx(1−2coshx) (D) 4ycoshxsinhx(1+2coshx)
›Reveal solutionSolution
Square both sides to remove the outer root, differentiate implicitly, then
tidy the resulting fraction — the answer comes out directly in terms of y
and coshx, matching option (D).
Concept and Intuition
When y is defined as a nested square root, it's usually easier to square first
(y2= the inside) and differentiate implicitly rather than applying the chain
rule twice directly to the nested radical — this avoids stacking two
2⋅1 factors and keeps the algebra manageable.
Step-by-Step Solution
- y=coshx+coshx ⇒ y2=coshx+coshx.
- Differentiate both sides w.r.t. x: 2ydxdy=sinhx+2coshx1⋅sinhx=sinhx(1+2coshx1).
- Combine the bracket over a common denominator: 1+2coshx1=2coshx2coshx+1.
- So 2yy′=2coshxsinhx(2coshx+1). …
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.If y=Tan−1x2−1+Sinh−1x2−1, x>1, then dxdy= (A) xx2−11 (B) xx2−1x+1 (C) x2x2−1x+1 (D) x2−1x
›Reveal solutionSolution
Both inverse-function derivatives share the same inner derivative
u′=x/x2−1; the 1+u2 under Tan−1 and 1+u2 under
Sinh−1 both simplify beautifully because u2=x2−1, so 1+u2=x2.
Concept and Intuition
Both Tan−1 and Sinh−1 have derivative formulas built around
1+u2 (as 1+u21 and 1+u21 respectively). Here
u=x2−1 makes 1+u2=x2 exactly, a clean perfect square — this is why
the two inverse functions are paired together in the problem, since they
combine so tidily.
Step-by-Step Solution
- Let u=x2−1. Then u′=x2−1x and 1+u2=1+(x2−1)=x2.
- dxdTan−1u=1+u2u′=x2x/x2−1=xx2−11.
- dxdSinh−1u=1+u2u′=x2x/x2−1=xx/x2−1=x2−11 (since x>1 means x2=x, not ∣x∣ ambiguity). …
- AP EAPCET 2026Set eng-2026-05-15-FN1 markMCQQ.If x=Sinh−1t+log(t2+1) and y=Tan−1t+log∣t∣, then dxdy= (A) 2t+t4+t2t2+t+1 (B) 2t+t2+1t2+t+1 (C) 2t2+t4+t2t2+t+1 (D) 2+1+t2t2+t+1
›Reveal solutionSolution
This tests parametric differentiation (dxdy=dx/dtdy/dt) with inverse-hyperbolic and inverse-trig terms; the answer is option (C).
Concept and Intuition
When both x and y are given in terms of a parameter t, we find dy/dx as the ratio of the two derivatives with respect to t, using dtdSinh−1t=1+t21 and dtdTan−1t=1+t21.
Step-by-Step Solution
- dtdx=1+t21+t2+12t (since dtdlog(t2+1)=t2+12t).
- dtdy=1+t21+t1=t(1+t2)t+(1+t2)=t(1+t2)t2+t+1.
- Write dtdx over common denominator 1+t2: dtdx=1+t21+t2+2t. …
- AP EAPCET 2024Set eng-2024-05-22-FN1 markMCQQ.If f(0)=0, f′(0)=3, then the derivative of y=f(f(f(f(f(x))))) at x=0 is (A) 16 (B) 32 (C) 81 (D) 243
›Reveal solutionSolution
Because 0 is a fixed point of f (f(0)=0), differentiating a repeated composition at x=0 just multiplies f′(0) by itself once per composition — five times here.
Concept and Intuition
By the chain rule, dxdf(g(x))=f′(g(x))g′(x). For a chain of five compositions, the derivative at a point is a product of five factors of f′, each evaluated at the running value of the inner composition at that point. Since f(0)=0, the running value stays 0 throughout, so every factor is just f′(0).
Step-by-Step Solution
- Let y=f(f(f(f(f(x))))) (five nested f's).
- By repeated chain rule: y′(x)=f′(f(f(f(f(x)))))⋅f′(f(f(f(x))))⋅f′(f(f(x)))⋅f′(f(x))⋅f′(x).
- At x=0: since f(0)=0, we get f(x)=0, then f(f(x))=f(0)=0, and so on — every nested value at x=0 is 0. …
- AP EAPCET 2022Set eng-2022-07-06-FN1 markMCQQ.If dxd(Alog(1−x3+11−x3+B))=x1−x31, then AB= (A) 31 (B) 3−1 (C) 3−2 (D) 32
›Reveal solutionSolution
Matching the derivative of a log-quotient expression to 1/(x1−x3) pins down A=1/3, B=−1, so AB=−1/3.
Concept and Intuition
This is a "guess the antiderivative form, then solve for constants" problem. Differentiate the given log expression symbolically in terms of u=1−x3, and choose B so the resulting denominator simplifies nicely (ideally to a pure power of x), then fix A by matching coefficients.
Step-by-Step Solution
- Let u=1−x3, so u′=21−x3−3x2=2u−3x2.
- dxd[Alog(u+1u+B)]=A[u+Bu′−u+1u′]=(u+B)(u+1)Au′(1−B).
- Try B=−1: then (u+B)(u+1)=(u−1)(u+1)=u2−1=(1−x3)−1=−x3.
- With B=−1, 1−B=2, so the expression becomes −x32Au′=x3−2A⋅2u−3x2=xu3A. …
- AP EAPCET 2025Set eng-2025-05-21-AN1 markMCQQ.If y=tanh−11+x1−x, then dxdy= (A) −21−x21 (B) −2x1−x21 (C) 1+x22 (D) 2x1+x21
›Reveal solutionSolution
Differentiating tanh−1u via the chain rule and simplifying u(1+x)=1−x2 gives dy/dx=−2x1−x21.
Concept and Intuition
tanh−1z=21log1−z1+z has derivative 1−z21, exactly like a standard log-based inverse function. Here z=u(x) is itself a composite square-root expression, so the chain rule applies twice; the algebra simplifies nicely because u2 is a simple rational function of x.
Step-by-Step Solution
- Let u=1+x1−x, so y=tanh−1u and dudy=1−u21.
- u2=1+x1−x⇒1−u2=1−1+x1−x=1+x(1+x)−(1−x)=1+x2x.
- Differentiate u2=1+x1−x w.r.t. x: 2udxdu=(1+x)2−(1+x)−(1−x)=(1+x)2−2⇒dxdu=u(1+x)2−1.
- By the chain rule: dxdy=dudy⋅dxdu=2x1+x⋅(u(1+x)2−1)=2xu(1+x)−1. …
- AP EAPCET 2025Set eng-2025-05-27-FN1 markMCQQ.If x=2cosec−1t and y=2sec−1t, ∣t∣≥1 then dxdy= (A) yx (B) xy (C) −xy (D) −yx
›Reveal solutionSolution
The identity cosec−1t+sec−1t=π/2 lets both x and y be written as exponentials of a single parameter u, so dy/dx follows from parametric differentiation. Answer: −xy.
Concept and Intuition
When x and y are both given as functions of a common (possibly hidden) parameter — here through the complementary inverse trig identity — the cleanest path is to introduce that parameter explicitly and use dxdy=dx/dudy/du, rather than trying to eliminate t directly.
Step-by-Step Solution
- For ∣t∣≥1, the standard identity cosec−1t+sec−1t=2π holds.
- Let u=cosec−1t. Then sec−1t=2π−u.
- x=2u=2u/2, and y=2π/2−u=2(π/2−u)/2=2π/4−u/2.
- Differentiate w.r.t. u: dudx=2u/2ln2⋅21=2xln2.
- dudy=2π/4−u/2ln2⋅(−21)=−2yln2. …
- AP EAPCET 2022Set eng-2022-07-07-FN1 markMCQQ.If ∫f(x)dx=F(x)+C, then dtdg(t)∫h(t)f(x)dx= (A) f(h(t))−f(g(t)) (B) F(h(t))−F(g(t)) (C) F(h(t))h′(t)−F(g(t))g′(t) (D) f(h(t))h′(t)−f(g(t))g′(t)
›Reveal solutionSolution
This is the Leibniz differentiation rule for integrals with variable limits.
Concept and Intuition
∫g(t)h(t)f(x)dx=F(h(t))−F(g(t)) where F′=f. Differentiating with respect to t needs the chain rule on both limits.
Step-by-Step Solution
- ∫g(t)h(t)f(x)dx=F(h(t))−F(g(t)), where F(x)=∫f(x)dx.
- Differentiate w.r.t. t: dtd[F(h(t))−F(g(t))]=F′(h(t))h′(t)−F′(g(t))g′(t).
- Since F′=f: this equals f(h(t))h′(t)−f(g(t))g′(t).
Common Mistakes …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.If y=Sec−1(2x1+x2) and x>1, then dxdy= (A) 1+x21 (B) 1+x22 (C) −1+x21 (D) −1+x22
›Reveal solutionSolution
Differentiating the inverse secant of a rational expression using the chain rule and careful algebraic simplification gives dxdy=1+x22.
Concept and Intuition
For y=Sec−1(u), the derivative formula is dxdy=∣u∣u2−11⋅dxdu. Here u=2x1+x2 is positive for x>1, so we can drop the absolute value and just carefully simplify the algebra — the key insight is recognizing that u2−1 factors as a perfect square-like expression involving (x2−1)2, which simplifies the square root beautifully.
Step-by-Step Solution
- Let u=2x1+x2. Compute dxdu using the quotient rule: u′=(2x)22x(2x)−(1+x2)(2)=4x24x2−2−2x2=4x22x2−2=2x2x2−1.
- Compute u2−1: u2−1=4x2(1+x2)2−4x2=4x2(1+x2−2x)(1+x2+2x)=4x2(x−1)2(x+1)2.
- So u2−1=2∣x∣∣x−1∣∣x+1∣=2∣x∣∣x2−1∣. For x>1: x2−1>0 and x>0, so u2−1=2xx2−1.
- For x>1, u=2x1+x2>0, so ∣u∣=u=2x1+x2. …
- AP EAPCET 2021Set eng-2021-08-24-FN1 markMCQQ.Find the value of 'k' if dxd⎩⎨⎧2+2+2+2cos(4x)2⎭⎬⎫=ksec(2x)tan(2x) (A) 21 (B) 2 (C) 1 (D) 81
›Reveal solutionSolution
Repeatedly apply the half-angle identity 2+2cosϕ=4cos2(ϕ/2) to peel away the nested square roots, collapsing the whole expression to a simple sec(x/2).
Concept and Intuition
The identity 1+cosϕ=2cos2(ϕ/2) (i.e. 2+2cosϕ=4cos2(ϕ/2)) is exactly designed to simplify nested square-root expressions of this kind — applying it repeatedly, from the innermost root outward, collapses the whole tower.
Step-by-Step Solution
- Innermost: 2+2cos4x=4cos2(2x) (using 2+2cosϕ=4cos2(ϕ/2) with ϕ=4x). So 2+2cos4x=2cos2x (taking cos2x>0).
- Next level: 2+2+2cos4x=2+2cos2x=4cos2x. So 2+2+2cos4x=2cosx.
- Next level: 2+2+2+2cos4x=2+2cosx=4cos2(x/2). So 2+2+2+2cos4x=2cos(x/2).
- So the whole expression is 2cos(x/2)2=sec(x/2).
- Differentiate: dxdsec(x/2)=sec(x/2)tan(x/2)⋅21. …
- AP EAPCET 2026Set eng-2026-05-18-AN1 markMCQQ.If y=Sech−1(9x2+109), then dxdy= (A) (9x2+10)2+81−18x (B) (9x2+10)2−81−18x (C) (9x2+19)(9x2+1)18x (D) (9x2+19)(9x2+1)18x(9x2+10)
›Reveal solutionSolution
A chain-rule differentiation of an inverse hyperbolic function, where (9x2+10)2−81 factors as a difference of squares into (9x2+1)(9x2+19).
Concept and Intuition
For y=sech−1u, the standard derivative is dudy=u1−u2−1 (for 0<u<1). The chain rule then just needs du/dx, and the algebra simplifies neatly because (9x2+10)2−92 is a difference of squares.
Step-by-Step Solution
- Let u=9x2+109. Then dxdu=9⋅(9x2+10)2−18x=(9x2+10)2−162x.
- 1−u2=1−(9x2+10)281=(9x2+10)2(9x2+10)2−81.
- Factor as a difference of squares: (9x2+10)2−92=(9x2+10−9)(9x2+10+9)=(9x2+1)(9x2+19).
- So 1−u2=9x2+10(9x2+1)(9x2+19).
- u1−u2=9x2+109⋅9x2+10(9x2+1)(9x2+19)=(9x2+10)29(9x2+1)(9x2+19). …
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