Q.Find dxdy in the following: y=cos−1(1+x21−x2),0<x<1
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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: …
Step 1: Let x=tanθ. Since 0<x<1, θ∈(0,4π). Then
1+x21−x2=1+tan2θ1−tan2θ=cos2θ,y=cos−1(cos2θ). …
Substituting x=tanθ simplifies y to 2tan−1x for 0<x<1; differentiating gives dxdy=1+x22.
Recognising the Identity
The expression 1+x21−x2 is the double-angle formula cos2θ=1+tan2θ1−tan2θ written in terms of x=tanθ. Substituting collapses the inverse cosine's argument to cos2θ, but simplifying cos−1(cos2θ) to 2θ requires 2θ to sit inside cos−1's principal range — which the given domain on x guarantees.
Step-by-Step Solution
1. Substitute x=tanθ.
Since 0<x<1, let θ=tan−1x∈(0,4π) (because tan0=0 and tan4π=1). Then:
1+x21−x2=1+tan2θ1−tan2θ=cos2θ⇒y=cos−1(cos2θ).
2. Check the range of 2θ.
Since θ∈(0,4π):
2θ∈(0,2π)⊂[0,π], …
Method: Simplifying a Composite Inverse-Cosine Expression via Substitution
This is the double-angle substitution technique applied to cos−1 — the key difference from the sin−1 version is that cos−1's principal range is [0,π], not [−2π,2π], so the branch-checking step uses a different interval.
Steps
Step 1: Recognise the double-angle-cosine shape
1+x21−x2 matches cos2θ=1+tan2θ1−tan2θ exactly.
Step 2: Substitute x=tanθ
The expression becomes y=cos−1(cos2θ), where θ=tan−1x.
Step 3: Work out the range of 2θ from the given domain of x
Convert the stated restriction on x into a restriction on θ=tan−1x (using that tan−1 is increasing), then double it to get the range of 2θ.
Step 4: Check that range against cos−1's principal range [0,π] — not [−2π,2π] …
Common Mistakes
Mistake 1: Checking the wrong principal range
Students who have just practised sin−1 problems often reflexively check whether the angle lies in [−2π,2π], forgetting that cos−1's principal range is different: [0,π]. Why it's wrong: using sine's range on a cosine problem can wrongly reject (or wrongly accept) a branch, leading to a sign or π-shift error. Correct approach: always identify which inverse function is on the outside first, and use its OWN principal range for the check.
Mistake 2: Forgetting to finish by differentiating the simplified y …
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: …
- CBSE 2026Set A1 markMCQQ.dxd(sinx2)=(a) 2xcosx2(b) cosx2(c) x2cosx2(d) xcosx2
›Reveal solutionSolution
dxdsin(x2)=2xcos(x2).
Let u=x2, so dxdu=2x.
By the chain rule, …
- CBSE 2026Set A1 markMCQQ.dxdcotx=(a) 2cotx1(b) csc2x(c) 2cotx−csc2x(d) 2cotxcsc2x
›Reveal solutionSolution
dxdcotx=2cotx−csc2x.
Let u=cotx, so dxdu=−csc2x.
Using dxdu=2u1⋅dxdu: …
- CBSE 2026Set A1 markMCQQ.dxd(cosx)=(a) sinx(b) 2x−sinx(c) 2xsinx(d) 2x1
›Reveal solutionSolution
dxdcosx=2x−sinx.
Let u=x, so dxdu=2x1.
By the chain rule, …
- CBSE 2026Set A1 markMCQQ.dxd(cosx3)=(a) −3x2sinx3(b) sinx3(c) 3x2sinx3(d) 3x2
›Reveal solutionSolution
Chain rule on cos(x3) gives −3x2sinx3.
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
›Reveal solutionSolution
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: …
- CBSE 2026Set ANNUAL1 markMCQQ.If y=cos−1(1+x21−x2), 0<x<1, then dxdy is equal to(a) 1+x21(b) 4+x2(c) 1+x22(d) x+x22
›Reveal solutionSolution
Put x=tanϕ so the expression simplifies to cos−1(cos2ϕ)=2ϕ=2tan−1x, whose derivative is standard.
Let x=tanϕ. Then 1+x21−x2=1+tan2ϕ1−tan2ϕ=cos2ϕ.
…
- CBSE 2026Set ANNUAL1 markQ.If y=ex+ex2+…+ex5, then find dxdy.
›Reveal solutionSolution
Differentiate each term exn using the chain rule: dxdexn=nxn−1exn.
y=ex+ex2+ex3+ex4+ex5
…
- CBSE 2026Set ANNUAL1 markMCQQ.dxd(cos3x)=(a) sin3x(b) −3sin3x(c) cos3x(d) −3cos3x
›Reveal solutionSolution
Differentiate cos(3x) using the chain rule: derivative of cosu is −sinu, times dxdu.
…
- CBSE 2026Set ANNUAL1 markMCQQ.dxdtan−1(x2)=(a) 1+x42x(b) 1+x2x(c) 1+x2x3(d) None of these
›Reveal solutionSolution
Use dxdtan−1u=1+u21⋅dxdu with u=x2.
…
- CBSE 2026Set ANNUAL1 markMCQQ.dxdesinx=(a) esinx⋅cosx(b) esinx(c) cosx(d) None of these
›Reveal solutionSolution
Differentiate eu with u=sinx: derivative is eu⋅dxdu.
…
- CBSE 2026Set ANNUAL1 markMCQQ.dxd(logsecx)=(a) tanx(b) cotx(c) cscx(d) None of these
›Reveal solutionSolution
Differentiate log(secx) using dxdlogu=uu′ with u=secx.
…
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