Q.Find dxdy in the following: logxcosx,x>0
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Derivative Evaluation
To evaluate a derivative means to find f′(a) — a single number that tells you how fast f is changing right at x=a. Think of a speedometer: it doesn't report how far you've travelled, only how fast your position is changing at this instant. That instantaneous rate of change is exactly what f′(a) measures.
The geometric picture
On the curve y=f(x), pick a point P and a nearby point Q. The straight line through them — the secant — has slope equal to the average rate of change between P and Q. Now slide Q toward P: the secant rotates into the tangent line that just touches the curve at P, and its slope is f′(a).
f′(a) is the slope of the tangent to y=f(x) at x=a — how steep the curve is right there.
The limit definition
f′(a)=limh→0hf(a+h)−f(a)
Here h is a tiny step from a to a+h, the numerator is the matching change in height, and the ratio is a secant slope. As h→0 the secant becomes the tangent. An equivalent form is
f′(a)=limx→ax−af(x)−f(a).
When this limit exists, f is differentiable at a (which forces continuity there).
Continuity alone is not enough. f(x)=∣x∣ is continuous at 0, but its left slope −1 and right slope +1 disagree, so f′(0) does not exist — a corner has no single tangent.
A worked evaluation
For f(x)=x2 at x=3:
f′(3)=limh→0h(3+h)2−9=limh→0(6+h)=6.
So the tangent at x=3 has slope 6.
From a number to a function …
The key idea is Implicit Differentiation — but here the function is given explicitly as y=logxcosx, so we simply differentiate using the quotient rule.
Let u=cosx and v=logx (natural log). Then:
dxdy=v2v⋅u′−u⋅v′
We have u′=−sinx and v′=x1. Substituting:
dxdy=(logx)2(logx)(−sinx)−(cosx)(x1) …
We use the quotient rule for differentiation because the function is a ratio of two differentiable functions. The derivative is dxdy=(logx)2−sinx⋅logx−xcosx.
The problem asks for dxdy of y=logxcosx, with x>0. The condition x>0 ensures the logarithm is defined and the denominator is non-zero (except at x=1, but we differentiate away from that point).
The core idea here is the quotient rule. Whenever you have a function that is one differentiable function divided by another, you don't need to rewrite it or use the product rule with a negative exponent (though that also works). The quotient rule is direct and clean.
The quotient rule: If y=vu, then dxdy=v2v⋅dxdu−u⋅dxdv.
Let’s apply it step by step.
-
Identify the numerator and denominator.
Here, u=cosx and v=logx. Both are differentiable for x>0.
-
Differentiate each part separately.
- dxdu=dxd(cosx)=−sinx
- dxdv=dxd(logx)=x1 (Remember: logx here means the natural logarithm, as is standard in calculus.)
-
Plug into the quotient rule formula.
dxdy=v2v⋅dxdu−u⋅dxdv=(logx)2(logx)(−sinx)−(cosx)(x1)
- Simplify the numerator. The numerator becomes −sinxlogx−xcosx. There’s no further algebraic simplification that makes it cleaner, so we leave it as is. …
Method: The Quotient Rule with a Logarithmic Denominator
Use this whenever y is a ratio where the denominator is a logarithm (or another standard function whose own derivative you must recall correctly).
Steps
Step 1: Identify the numerator and denominator functions
Write y=vu with u=cosx and v=logx, valid for x>0 (so the logarithm is defined) and excluding x=1 (where logx=0 would make the denominator zero).
Step 2: Recall the quotient rule formula
dxdy=v2vdxdu−udxdv
Step 3: Differentiate u and v separately, using the correct standard derivative for each
dxdu=−sinx,dxdv=x1 …
Common Mistakes
Mistake 1: Misremembering the derivative of logx
Why it's wrong: writing dxdlogx=logx1 instead of x1 is one of the most frequent errors with logarithmic denominators — it conflates the function's value with its derivative. Correct approach: fix dxdlogx=x1 firmly and double-check it every time a logarithm appears in a quotient.
Mistake 2: Swapping the order of the two terms in the quotient rule numerator
Why it's wrong: the formula is vu′−uv′, and reversing it to uv′−vu′ flips the overall sign. Correct approach: always write "denominator times derivative of numerator, minus numerator times derivative of denominator" before substituting. …
Showing the 12 most recent of 48 on this concept.
- AP EAPCET 2026Set eng-2026-05-12-FN1 markMCQQ.Let f(x)=log(4x5)+x3e−x1. If f′(x)=x3e−x1G(x)+xk, then the roots of G(x)+k=0 is (A) 43,1 (B) 1,−3 (C) 52,2 (D) 2,43
›Reveal solutionSolution
Differentiate f(x) term by term, match the given form to identify G(x) and k, then solve the resulting quadratic G(x)+k=0. Roots: 52 and 2.
Concept and Intuition
The function is a sum of a log term and a product term. Differentiating the product term using the product rule and factoring out x3e−1/x (matching the form given in the problem) isolates G(x) directly by comparison; the log term's derivative isolates k.
Step-by-Step Solution
- Rewrite f(x)=log(x5/4)+x3e−1/x=45logx+e−1/xx−3.
- Differentiate the log term: dxd(45logx)=4x5.
- Differentiate the product term using the product rule. First, dxde−1/x=e−1/x⋅dxd(−x1)=e−1/x⋅x21. dxd(e−1/xx−3)=x2e−1/x⋅x−3+e−1/x⋅(−3x−4)=e−1/x(x−5−3x−4).
- Factor out x3e−1/x: e−1/x(x−5−3x−4)=x3e−1/x(x−2−3x−1)=x3e−1/x(x21−x3). …
- AP EAPCET 2026Set eng-2026-05-13-FN1 markMCQQ.If y=sinxcosxxcosx−sinx1sinxcosx1, then dxdy= (A) xcosx (B) xsinx (C) sinx+cosx (D) 1
›Reveal solutionSolution
Expanding the determinant gives y=x−1, so dxdy=1 — option (D).
Working. Expand along the first row:
y=sinx[(−sinx)(1)−(cosx)(1)]−cosx[(cosx)(1)−(cosx)(x)]+sinx[(cosx)(1)−(−sinx)(x)]
=−sin2x−sinxcosx−cos2x(1−x)+sinxcosx+xsin2x
The two sinxcosx terms cancel, leaving: …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.x→3πlimcos(x+6π)tan3x−3tanx= (A) 12 (B) 24 (C) −24 (D) −12
›Reveal solutionSolution
A 0/0 trig limit resolved by L'Hôpital's Rule; the answer is −24.
Concept and Intuition
Both numerator and denominator vanish at x=π/3 (since tan(π/3)=3 makes tan3x−3tanx=0, and x+π/6=π/2 makes cos(x+π/6)=0). This is a genuine 0/0 form, so we differentiate numerator and denominator with respect to x and substitute.
Step-by-Step Solution
- Numerator N(x)=tan3x−3tanx. Its derivative: N′(x)=3tan2xsec2x−3sec2x=3sec2x(tan2x−1).
- At x=π/3: sec(π/3)=2⇒sec2=4; tan2(π/3)=3⇒tan2x−1=2. So N′(π/3)=3(4)(2)=24.
- Denominator D(x)=cos(x+π/6). Its derivative: D′(x)=−sin(x+π/6).
- At x=π/3: x+π/6=π/2, and sin(π/2)=1, so D′(π/3)=−1. …
- AP EAPCET 2026Set eng-2026-05-14-AN1 markMCQQ.If y=Tan−1(1+x+x21)+Tan−1(x2+3x+31)+Tan−1(x2+5x+71), then y′(0)= (A) −103 (B) −21 (C) −107 (D) −109
›Reveal solutionSolution
Each arctan term telescopes using arctanA−arctanB=arctan1+ABA−B, collapsing the whole sum to arctan(x+3)−arctanx; differentiating and plugging x=0 gives −9/10.
Concept and Intuition
The identity arctanA−arctanB=arctan1+ABA−B (when AB>−1) is the key: if we can write each denominator 1+x+x2 etc. as 1+AB for consecutive integers-shifted A,B with A−B=1, the arctan of the reciprocal collapses to a difference of two arctans. Stacking three such differences telescopes almost everything away, leaving only the first and last terms.
Step-by-Step Solution
- First term: want A−B=1, AB=x+x2=x(x+1). Take A=x+1,B=x: A−B=1 ✓, AB=x(x+1)=x2+x ✓. So arctan1+x+x21=arctan(x+1)−arctanx.
- Second term: want AB=x2+3x+2=(x+1)(x+2). Take A=x+2,B=x+1: AB=(x+1)(x+2) ✓. So arctanx2+3x+31=arctan(x+2)−arctan(x+1).
- Third term: want AB=x2+5x+6=(x+2)(x+3). Take A=x+3,B=x+2. So arctanx2+5x+71=arctan(x+3)−arctan(x+2).
- Sum: y=[arctan(x+1)−arctanx]+[arctan(x+2)−arctan(x+1)]+[arctan(x+3)−arctan(x+2)]. …
- AP EAPCET 2026Set eng-2026-05-15-AN1 markMCQQ.If f(x)=Tan−1(1+x2−1−x21+x2+1−x2), then f′(−21)= (A) −21 (B) 21 (C) −152 (D) 152
›Reveal solutionSolution
This tests simplifying an inverse-trig expression via a trigonometric substitution before differentiating. Once simplified, f(x)=4π+21cos−1(x2), giving f′(−21)=152.
Concept and Intuition
Expressions with 1+x2 and 1−x2 together strongly suggest substituting x2=cosφ, turning both square roots into half-angle sine/cosine forms via 1±cosφ=2cos22φ or 2sin22φ. This collapses the arctangent of a ratio into a simple tangent addition, making the function (and its derivative) far easier to handle than direct differentiation of the original expression.
Step-by-Step Solution
- Let x2=cosφ. Then 1+x2=1+cosφ=2cos22φ and 1−x2=1−cosφ=2sin22φ.
- So 1+x2=2cos2φ and 1−x2=2sin2φ (taking the principal positive roots).
- The ratio inside f: cos2φ−sin2φcos2φ+sin2φ=1−tan2φ1+tan2φ=tan(4π+2φ).
- So f(x)=tan−1[tan(4π+2φ)]=4π+2φ=4π+21cos−1(x2). …
- AP EAPCET 2026Set eng-2026-05-15-FN1 markMCQQ.If f(x)=−(1+x) Sec−1x is a real valued function, then f′(x)= (A) −2−(1+x)Sec−1x+xx−11 (B) −2−(1+x)Sec−1x−x1−x1 (C) −2−(1+x)Sec−1x−xx−11 (D) −2−(1+x)Sec−1x+x1−x1
›Reveal solutionSolution
This tests differentiating a product involving Sec−1x on its x≤−1 branch, being careful with the sign of ∣x∣ inside the arcsec derivative; the answer is option (B).
Concept and Intuition
f is only real for x≤−1 (so that −(1+x)≥0 and ∣x∣≥1). On this branch, x2−1 can be split as 1−x⋅−1−x — both factors positive when x≤−1 — and −1−x is exactly the u already in the problem, which lets the answer be written in the given form.
Step-by-Step Solution
- Let u=−(1+x), v=Sec−1x, so f=uv and f′=u′v+uv′.
- u′=2−(1+x)1⋅(−1)=−2u1.
- Standard result: dxdSec−1x=∣x∣x2−11. For x≤−1, ∣x∣=−x, so v′=−xx2−11.
- For x≤−1: x2−1=(1−x)(−1−x)=1−x⋅−1−x=1−x⋅u.
- So v′=−x1−xu1. …
- AP EAPCET 2025Set eng-2025-05-21-FN1 markMCQQ.If α and β (α>β) are the multiple roots of the equation 4x4+4x3−23x2−12x+36=0, then 2α−β= (A) −1 (B) 3 (C) 5 (D) −7
›Reveal solutionSolution
The quartic factors neatly as (x+2)2(2x−3)2, so its two repeated ('multiple') roots are −2 and 23; with α>β, 2α−β=5.
Concept and Intuition
'Multiple roots' of a polynomial are roots with multiplicity greater than 1 (i.e., repeated roots). A quartic with two distinct double roots factors as a(x−α)2(x−β)2 — recognizing this pattern is far faster than blindly applying the quartic formula.
Step-by-Step Solution
- Try small integer/rational candidates in 4x4+4x3−23x2−12x+36=0. Testing x=−2: 4(16)+4(−8)−23(4)−12(−2)+36=64−32−92+24+36=0. So x=−2 is a root.
- Synthetic division by (x+2): 4x4+4x3−23x2−12x+36÷(x+2)=4x3−4x2−15x+18.
- Test x=−2 again in this cubic: 4(−8)−4(4)−15(−2)+18=−32−16+30+18=0. So x=−2 is a repeated root (multiplicity at least 2).
- Divide again by (x+2): 4x3−4x2−15x+18÷(x+2)=4x2−12x+9.
- Factor the quadratic: 4x2−12x+9=(2x−3)2, giving the repeated root x=23 (also multiplicity 2, since it's a perfect square).
- So the full factorization is 4x4+4x3−23x2−12x+36=(x+2)2(2x−3)2 (verified by re-expanding: (x2+4x+4)(4x2−12x+9)=4x4+4x3−23x2−12x+36 ✓). …
- AP EAPCET 2025Set eng-2025-05-21-FN1 markMCQQ.The derivative of Sec−1(2x2−11) with respect to 1−x2 at x=21 is (A) −2 (B) 1 (C) 2 (D) 4
›Reveal solutionSolution
A "derivative with respect to another function" problem — the substitution x=cosθ collapses Sec−1(2x2−1)−1 into 2θ, making the ratio of derivatives trivial. Answer: 4.
Concept and Intuition
To find dvdu where u=u(x) and v=v(x), use dvdu=dv/dxdu/dx (or, more cleanly here, a common parameter θ). The key trick recognizing 2x2−1 as cos2θ when x=cosθ turns the inverse secant of a rational expression into a simple linear function of θ.
Step-by-Step Solution
- Let x=cosθ, θ∈[0,π] (the natural domain for cos−1).
- Then 2x2−1=2cos2θ−1=cos2θ, so 2x2−11=cos2θ1=sec2θ.
- Hence u=Sec−1(sec2θ). Since x=21⇒θ=cos−1(21)=3π, we get 2θ=32π, which lies in [0,π] — the principal range of Sec−1 — so u=2θ=2cos−1x validly (no branch correction needed here).
- Also v=1−x2=1−cos2θ=sinθ (non-negative since θ∈[0,π]).
- Differentiate w.r.t. θ: dθdu=2, dθdv=cosθ=x.
- So dvdu=dv/dθdu/dθ=x2.
- At x=21: dvdu=1/22=4. …
- AP EAPCET 2025Set eng-2025-05-22-AN1 markMCQQ.If the tangent drawn at the point (x1,y1), x1,y1∈N on the curve y=x4−2x3+x2+5x passes through origin, then x1+y1= (A) 5 (B) 4 (C) 7 (D) 6
›Reveal solutionSolution
The tangent-through-origin condition gives an algebraic equation in x1 that factors cleanly; the only natural-number root is x1=1, giving y1=5 and x1+y1=6.
Concept and Intuition
If the tangent at (x1,y1) on a curve y=f(x) passes through the origin, the slope of the tangent (which is f′(x1)) must also equal the slope of the line from the origin to (x1,y1), i.e. x1−0y1−0=x1y1. This gives y1=x1f′(x1), a clean algebraic condition combining the curve's value and its derivative at the same point.
Step-by-Step Solution
- f(x)=x4−2x3+x2+5x, so y1=f(x1)=x14−2x13+x12+5x1.
- f′(x)=4x3−6x2+2x+5, so the tangent slope at x1 is f′(x1)=4x13−6x12+2x1+5.
- Tangent line through (x1,y1) passing through the origin: slope =x1y1=f′(x1) (for x1=0), i.e. y1=x1f′(x1):
x14−2x13+x12+5x1=x1(4x13−6x12+2x1+5)=4x14−6x13+2x12+5x1
- Simplify:
0=(4x14−6x13+2x12+5x1)−(x14−2x13+x12+5x1)=3x14−4x13+x12
=x12(3x12−4x1+1)=x12(3x1−1)(x1−1) …
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.The slope of a tangent drawn at the point P(α,β) lying on the curve y=2x−51 is −2. If P lies in the fourth quadrant, then α−β= (A) 4 (B) 3 (C) 2 (D) 1
›Reveal solutionSolution
Set the derivative equal to −2 to get two candidate x-values, then use the
fourth-quadrant condition (x>0, y<0) to pick the right one: α−β=3.
Concept and Intuition
The slope condition alone gives a quadratic (two candidate points on the curve),
since dxdy=−2/(2x−5)2 is even in (2x−5). The extra geometric
condition — "P lies in the fourth quadrant" — is exactly what's needed to
discard the spurious root and pin down a unique point.
Step-by-Step Solution
- y=2x−51=(2x−5)−1, so dxdy=−2(2x−5)−2=(2x−5)2−2.
- Set slope =−2: (2x−5)2−2=−2 ⇒ (2x−5)2=1 ⇒ 2x−5=±1.
- 2x−5=1⇒x=3; 2x−5=−1⇒x=2.
- At x=3: y=2(3)−51=11=1. Point (3,1) — first quadrant (x>0,y>0), rejected. …
- AP EAPCET 2025Set eng-2025-05-24-FN1 markMCQQ.If y=Tan−1(1−3x23x−x3)+Tan−1(1−12x27x), then at x=0, dxdy= (A) 6 (B) 7 (C) 9 (D) 10
›Reveal solutionSolution
Differentiate the sum of two arctangent expressions term-by-term and evaluate at x=0; the answer is 10.
Concept and Intuition
Rather than trying to recognize the whole expression as some multiple-angle identity, it is safest and fastest to differentiate each Tan−1(⋅) term directly using the chain rule dxdTan−1(u)=1+u2u′, then substitute x=0. Since we only need the derivative at a point, we don't need the general antiderivative simplification (e.g., recognizing 1−3x23x−x3 as tan(3θ) for x=tanθ) — direct differentiation is more robust.
Step-by-Step Solution
- Let y=Tan−1(u)+Tan−1(v) where u=1−3x23x−x3 and v=1−12x27x.
- First term derivative at x=0:
u′=(1−3x2)2(3−3x2)(1−3x2)−(3x−x3)(−6x).
At x=0: numerator =(3)(1)−0=3, denominator =1, so u′(0)=3. Also u(0)=0.
Contribution: 1+u(0)2u′(0)=13=3.
3. Second term derivative at x=0:
v′=(1−12x2)27(1−12x2)−7x(−24x).
At x=0: numerator =7(1)−0=7, denominator =1, so v′(0)=7. Also v(0)=0. …
- AP EAPCET 2025Set eng-2025-05-24-FN1 markMCQQ.If y=(x2−3)(2x−3)x43x−5, then (dxdy)x=2= (A) 5 (B) 0 (C) 1 (D) −5
›Reveal solutionSolution
Logarithmic differentiation converts the messy root/product/quotient into a sum of logs; evaluating at x=2 gives y′=−5.
Concept and Intuition
Whenever a function is a product, quotient, or power of several simpler factors (especially under a square root), logarithmic differentiation is the cleanest route: take log of both sides to convert products to sums and powers to multiples, differentiate termwise, then multiply back by y.
Step-by-Step Solution
- Write y=[(x2−3)(2x−3)x43x−5]1/2. Taking log:
logy=21[4logx+21log(3x−5)−log(x2−3)−log(2x−3)].
- Differentiate both sides with respect to x:
yy′=21[x4+21⋅3x−53−x2−32x−2x−32].
- Evaluate the original y at x=2: x4=16, 3x−5=1⇒1=1, x2−3=1, 2x−3=1. So y=16⋅1/(1⋅1)=16=4.
- Evaluate each bracket term at x=2:
- x4=24=2
- 21⋅3x−53=21⋅13=1.5 …
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