Q.Differentiate the following with respect to x: sin(tan−1e−x)
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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 — simplify sin(tan−1u)=1+u2u first, then differentiate.
With u=e−x: y=1+e−2xe−x=e−x(1+e−2x)−1/2.
Using the product rule with p=e−x, q=(1+e−2x)−1/2: p′=−e−x, q′=e−2x(1+e−2x)−3/2. …
Simplifying sin(tan−1e−x) using a right-triangle identity before differentiating gives dxdy=−(1+e−2x)3/2e−x.
Differentiating y=sin(tan−1e−x) directly (chain rule through sine, then arctan, then the exponential) is possible but algebraically messy. A cleaner route is to first simplify the composition sin(tan−1u) using a right triangle.
Step 1 — Simplify the inner composition.
For any real u, if θ=tan−1u then tanθ=u; picture a right triangle with opposite side u and adjacent side 1, so the hypotenuse is 1+u2. Then sinθ=1+u2u, i.e.
sin(tan−1u)=1+u2u.
With u=e−x,
y=1+e−2xe−x=e−x(1+e−2x)−1/2.
Step 2 — Differentiate using the product rule.
Let p=e−x and q=(1+e−2x)−1/2, so y=pq and y′=p′q+pq′.
- p′=−e−x.
- For q, apply the chain rule: q′=−21(1+e−2x)−3/2⋅dxd(1+e−2x)=−21(1+e−2x)−3/2⋅(−2e−2x)=e−2x(1+e−2x)−3/2.
Step 3 — Combine. …
Method: Simplify a Trig-of-Inverse-Trig Composition with a Right-Triangle Identity Before Differentiating
Use this whenever you must differentiate an expression of the form sin(tan−1u), cos(tan−1u), tan(sin−1u), etc. — direct chain-rule differentiation is possible but produces messy sec2/ expressions that are easy to mismanage.
Steps
Step 1: Build the reference right triangle for the inner inverse trig function
For θ=tan−1u, picture a right triangle with opposite side u, adjacent side 1, hypotenuse 1+u2. This converts the inverse-trig angle into concrete side ratios.
Step 2: Read off the needed trig ratio from the triangle
sin(tan−1u)=1+u2u
This identity holds for all real u and eliminates the inverse trig function completely.
Step 3: Substitute the given inner function for u …
Common Mistakes
Mistake 1: Differentiating sin(tan−1e−x) directly, layer by layer, without simplifying first
Why it's wrong: this route works in principle but forces you to carry cos(tan−1e−x) and 1+e−2x1 through several more algebra steps, multiplying the chance of a sign or simplification error. Correct approach: convert sin(tan−1u) to 1+u2u first using the right-triangle identity, then differentiate the resulting algebraic expression.
Mistake 2: Dropping the negative sign when differentiating e−x
Why it's wrong: dxde−x=−e−x, not e−x — missing this sign flips the sign of every subsequent term. Correct approach: always write the chain rule factor explicitly: dxde−x=e−x⋅(−1). …
Showing the 12 most recent of 115 on this concept.
- CBSE 2026Set A1 markMCQQ.dxdx2+ax+1=(a) 2x2+ax+1x+a(b) 2x2+ax+12x+a(c) x2+ax+12x+a(d) 2x2+ax+11
›Reveal solutionSolution
dxdx2+ax+1=2x2+ax+12x+a.
Let u=x2+ax+1, so dxdu=2x+a.
Using dxdu=2u1⋅dxdu: …
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Let u=x2, so dxdu=2x.
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dxdcotx=2cotx−csc2x.
Let u=cotx, so dxdu=−csc2x.
Using dxdu=2u1⋅dxdu: …
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By the chain rule, …
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Let the inner function be u=x3, so y=cosu.
…
- CBSE 2026Set ANNUAL1 markMCQQ.Write the derivative of log(cosex).(a) −tanex(b) extanex(c) −extanex(d) tanex
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Using the chain rule twice, dxdlog(cosex)=−extanex.
Let y=log(cosex). We differentiate using the chain rule, working from the outside in.
Step 1: Differentiate log(u) where u=cosex:
dxdy=cosex1⋅dxd(cosex)
Step 2: Differentiate cos(v) where v=ex: …
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Put x=tanϕ so the expression simplifies to cos−1(cos2ϕ)=2ϕ=2tan−1x, whose derivative is standard.
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…
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y=ex+ex2+ex3+ex4+ex5
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…
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…
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