Q.Find dxdy in the following: ex+ex2+...+ex5
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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 — each term exn differentiates to nxn−1exn; sum the results. …
Differentiating each exponential term with the chain rule and adding the results gives dxdy=ex+2xex2+3x2ex3+4x3ex4+5x4ex5.
The pattern in the question continues through powers x,x2,x3,x4,x5 — five terms in total, each of the form exn. Since the derivative of a sum is the sum of the derivatives, differentiate each term separately using the chain rule: for ef(x), the derivative is ef(x)⋅f′(x).
Term 1: ex. Inner function x, derivative 1. dxdex=ex.
Term 2: ex2. Inner function x2, derivative 2x. dxdex2=2xex2.
Term 3: ex3. Inner function x3, derivative 3x2. dxdex3=3x2ex3.
Term 4: ex4. Inner function x4, derivative 4x3. dxdex4=4x3ex4. …
Method: Differentiating a Sum of Terms of the Form exn
Use this whenever y is a sum of several exponential terms, each with a different power of x in the exponent.
Steps
Step 1: Split the sum into individual terms
Since the derivative of a sum is the sum of the derivatives, differentiate each term exn completely independently before adding the results back together.
Step 2: For each term, identify the inner function
For a term exn, the outer function is eu and the inner function is u=xn.
Step 3: Apply the chain rule to each term
dxdexn=exn⋅dxd(xn)=nxn−1exn …
Common Mistakes
Mistake 1: Forgetting the inner-derivative factor for higher-power terms
Why it's wrong: writing dxdex3=ex3 (missing the 3x2 factor) is a very common slip once several similar terms are stacked in one problem. Correct approach: for every term of the form exn, explicitly write the factor nxn−1 before moving to the next term.
Mistake 2: Mixing up the exponent power with the coefficient
Why it's wrong: for ex4 the correct factor is 4x3 (power drops by one), not 4x4 or x3 alone — losing either the coefficient or the reduced power gives a wrong term. Correct approach: apply the ordinary power rule to the exponent xn exactly as you would to any polynomial term, then attach that as a multiplier to exn. …
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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