Q.Find dxdy in the following: y=sin−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=cos2θ,y=sin−1(cos2θ)=sin−1[sin(2π−2θ)]. …
Substituting x=tanθ simplifies y to 2π−2tan−1x for 0<x<1; differentiating gives dxdy=−1+x22.
Recognising the Identity
As in the neighbouring cos−1 version of this expression, 1+x21−x2=cos2θ under x=tanθ. Here the outer function is sin−1, so cos2θ must first be rewritten as a sine using the complementary-angle identity before the inverse sine can cancel it.
Step-by-Step Solution
1. Substitute x=tanθ.
Since 0<x<1, let θ=tan−1x∈(0,4π). Then:
1+x21−x2=1+tan2θ1−tan2θ=cos2θ⇒y=sin−1(cos2θ).
2. Rewrite the cosine as a sine.
Using cosϕ=sin(2π−ϕ) with ϕ=2θ:
cos2θ=sin(2π−2θ)⇒y=sin−1[sin(2π−2θ)].
3. Check the range.
Since θ∈(0,4π), 2θ∈(0,2π), so:
2π−2θ∈(0,2π)⊂[−2π,2π], …
Method: Converting Between Inverse-Sine and Inverse-Cosine Forms via the Complementary Identity
This method is needed when the double-angle substitution naturally produces a cosine (like cos2θ), but the outer inverse function is sin−1, not cos−1 — you must convert cosine to sine using the complementary-angle identity before you can cancel.
Steps
Step 1: Substitute and identify which trig function you land on
With x=tanθ, 1+x21−x2=cos2θ, so the expression is y=sin−1(cos2θ) — a cosine trapped inside an inverse sine.
Step 2: Rewrite the cosine as a sine using the complementary identity
cosϕ=sin(2π−ϕ).
Applying this with ϕ=2θ gives cos2θ=sin(2π−2θ), so y=sin−1[sin(2π−2θ)] — now the inner and outer functions match.
Step 3: Check the new angle against sin−1's range [−2π,2π] …
Common Mistakes
Mistake 1: Applying sin−1(cosα)=α directly, without converting
Because the inner expression naturally simplifies to cos2θ, it's tempting to plug it straight into sin−1 and cancel, as if sin−1(cosα)=α were a valid identity. Why it's wrong: sin−1 only undoes sin, not cos — you must first rewrite cos2θ as sin(2π−2θ) using the complementary-angle identity before the inverse sine can cancel it. Correct approach: always match the trig function inside the expression to the SAME trig function as the outer inverse before cancelling; convert first if they don't match.
Mistake 2: Mishandling the sign when converting via the complementary identity …
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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