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GATE DA

Limits, continuity and differentiability

This topic covers how a function of one real variable behaves near a point: whether a limit exists, whether the function is continuous there, and whether it has a derivative. GATE questions test standard limits, L'Hopital's rule, parameters that make a piecewise function continuous or differentiable, and the mean value theorems, and all of them rest on the definitions and the one-way link that differentiability implies continuity.

6 previous-year questions · 6 solved · 5 practice questions

Limits and the standard results

Convention: all functions here are real-valued functions of one real variable, is the natural logarithm and angles are in radians. The limit (with finite) means gets arbitrarily close to as gets close to (from both sides), whatever itself is, or even if is undefined. At an endpoint of the domain only the one-sided limit is used.

LIMIT EXISTS IFF BOTH ONE-SIDED LIMITS AGREE

Use one-sided limits for , floor, piecewise functions and any function that changes formula at .

LIMIT LAWS

Valid when and both exist and are finite.

STANDARD TRIGONOMETRIC LIMITS

Only for in radians and . For , instead.

STANDARD EXPONENTIAL AND LOGARITHMIC LIMITS

Each is a form that equals a derivative at (of , and respectively).

LIMITS DEFINING E AND THE BINOMIAL LIMIT

Here is any real number. More generally for any real .

THE $1^\INFTY$ FORM

Applies when near , , , and the limit inside the exponential exists.

EXPANSIONS NEAR $X=0$: EXPONENTIAL AND LOGARITHM

The series holds for all real ; the series holds for .

EXPANSIONS NEAR $X=0$: TRIGONOMETRIC

The and series hold for all real ; the series for . In a limit at , replace each function by enough terms and compare the lowest powers: for example .

Other tools: factorise and cancel for with polynomials, multiply by the conjugate for square-root differences, divide by the highest power for , and use the squeeze theorem: if near and then . For example as because .

Indeterminate forms and L'Hopital's rule

The forms , , , , , and do not have a value decided by the pieces alone. Reduce each to or and then apply L'Hopital's rule, or use a standard limit or a series expansion.

L'HOPITAL'S RULE

Needs: of the form or as ; differentiable with on a punctured neighbourhood of ; and the limit of (the limit on the right of the equation) to exist, finite or infinite. It also holds for one-sided limits and for . Differentiate numerator and denominator separately; do not use the quotient rule.

Reducing other forms

  1. : write as or to get or .

  2. : combine into one fraction, or multiply and divide by the conjugate.

  3. , , (base positive near ): take of the expression, find the limit of the logarithm by L'Hopital's rule, then the answer is .

  4. Repeat L'Hopital's rule only while the form remains or .

GROWTH ORDER

Exponentials beat powers and powers beat logarithms as .

WORKED EXAMPLE

Find .

Solution
  1. The form is .

  2. Write it as .

  3. Since , the whole expression tends to .

  4. Check with the rule: , so the limit is .

Answer:

EXAM TIP

Before using L'Hopital's rule, confirm the form really is or . Applying it to a determinate form, such as , gives a wrong answer. Also, if the limit of does not exist, the rule says nothing about .

Continuity and types of discontinuity

CONTINUITY AT A POINT

Three things must hold: is defined, the limit exists and is finite, and the two are equal. Equivalently, left limit = right limit = .

A function is continuous on an interval if it is continuous at every point of it (at an endpoint, use the one-sided limit). Sums, differences, products and quotients (where the denominator is nonzero) of functions continuous at are continuous at ; a composition is continuous at when is continuous at and is continuous at . Polynomials, , , , are continuous everywhere, on , wherever .

In the table below, following common usage in GATE questions, a point where is undefined but defined on both sides near it is also classified as a discontinuity.

Type

What happens at

Example at

Removable

exists and is finite but differs from , or is undefined

(undefined at , limit )

Jump

Both one-sided limits exist and are finite but are unequal

(limits and )

Infinite

At least one one-sided limit is

Oscillating

At least one one-sided limit does not exist and is not infinite

INTERMEDIATE VALUE THEOREM

Used to show a root exists, for example gives a root of in .

EXTREME VALUE THEOREM

Both conditions matter: on the open interval , is continuous but has no maximum.

EXAM TIP

To make a piecewise function continuous at the join point, set left limit = right limit = value there. That gives one equation per join. Do not forget to evaluate the one-sided limits using the correct formula on each side.

Differentiability and first principles

DERIVATIVE FROM FIRST PRINCIPLES

is differentiable at when this limit exists and is finite.

LEFT AND RIGHT DERIVATIVES

is differentiable at if and only if both exist, are finite and .

DIFFERENTIABILITY IMPLIES CONTINUITY

The converse is false. Hence a function that is not continuous at cannot be differentiable at .

The standard counterexample is : it is continuous at , but and , so it is not differentiable there. Common ways a function fails to be differentiable at a point include a corner (unequal one-sided slopes, e.g. at ), a vertical tangent (slope , e.g. at ), a cusp (one-sided slopes and , e.g. at ), oscillation (e.g. with , continuous at but has no limit), and a discontinuity.

Function

At

Reason

continuous, not differentiable

corner: slopes and

continuous, not differentiable

vertical tangent, slope

differentiable,

,

differentiable, , but is not continuous at

has no limit as

WORKED EXAMPLE

Find constants and so that for and for is differentiable at .

Solution
  1. Differentiability needs continuity first. Left value: . Right limit: . So .

  2. Left derivative at : at is . Right derivative (using ): . So and .

  3. Then .

Answer:

Rules and standard derivatives

PRODUCT AND QUOTIENT RULES

For differentiable at the point.

CHAIN RULE

For differentiable at and differentiable at .

Domain note

positive integer : all ; negative integer : ; other real :

all

,

,

all

all

LOGARITHMIC DIFFERENTIATION

For and differentiable. For example for .

EXAM TIP

For with differentiable, differentiability can fail only where . If and , has a corner at (slopes ). If and (for a polynomial, a repeated root), is still differentiable at with derivative , because .

Rolle's and the mean value theorems

ROLLE'S THEOREM

All three conditions are needed. Typical use: if is continuous on the closed interval between two roots and differentiable inside it, then has a root strictly between them.

LAGRANGE'S MEAN VALUE THEOREM

Some point has tangent slope equal to the average slope over . Rolle's theorem is the special case .

CAUCHY'S MEAN VALUE THEOREM

The condition ensures (by Rolle's theorem). Taking gives Lagrange's theorem.

Consequences and how to use them

  1. If is differentiable on an interval and for all in it, then is constant there.

  2. If is differentiable on an interval and there, is strictly increasing there; if , strictly decreasing. The converse fails: is strictly increasing but .

  3. To find , solve and keep only solutions strictly inside .

  4. The conditions are sufficient, not necessary: a function can have such a even if a condition fails, but failing a condition removes the guarantee.

WORKED EXAMPLE

Verify Lagrange's mean value theorem for on and find .

Solution
  1. is a polynomial, so it is continuous on and differentiable on .

  2. Average slope: .

  3. gives , which lies in .

Answer:

EXAM TIP

Rolle's theorem can fail when differentiability fails at one interior point. For on we have , yet wherever it exists, because the only candidate is a corner.

PREVIOUS-YEAR QUESTIONS

6 questions from the DA papers

Try each question first, then open the answer. We placed each question under a topic ourselves; the exam paper does not label topics.

See these papers in full: GATE DA 2025 · GATE DA 2024

GATE 2024 DA Q.37

GATE 2024 · DAQ.37 · 2 marks · MCQ

Let be a function. Note: denotes the set of real numbers.

Which ONE of the following choices gives the values of that make the function continuous and differentiable?

  1. (A)

  2. (B)

  3. (C)

  4. (D)

Answer and solution
See the question in the paper
GATE 2024 DA Q.37: answer and solution

PRACTICE

5 questions to test yourself

PRACTICE 11 mark · NAT

Evaluate . Give the value as a decimal.

Answer and solution

Answer: (accepted range )

  1. At the form is , so apply L'Hopital's rule.

  2. Differentiate: numerator , denominator . This is still .

  3. Differentiate again: numerator , denominator .

  4. At this is .

PRACTICE 22 marks · MCQ

The number of points in at which is not differentiable is

  1. (A)

    0

  2. (B)

    1

  3. (C)

    2

  4. (D)

    3

Answer and solution

Answer: (C)

  1. can fail to be differentiable only where .

  2. , so the zeros are and .

  3. At each, equals and , which are nonzero, so has a corner at both.

  4. Hence exactly 2 points.

PRACTICE 32 marks · MSQ

Let for and . Which of the following statements are TRUE?

  1. (A)

    is continuous at

  2. (B)

    is differentiable at

  3. (C)

    is continuous at

  4. (D)

Answer and solution

Answer: (A), (B)

  1. , so and is continuous: A is true.

  2. , and , so exists: B is true and D is false.

  3. For , . The first term tends to but oscillates, so has no limit at and is not continuous there: C is false.

PRACTICE 41 mark · NAT

The function for , with , is continuous at . Find .

Answer and solution

Answer: (accepted range )

  1. Continuity requires .

  2. Write .

  3. As , and , so the limit is .

PRACTICE 52 marks · NAT

For on the interval , Lagrange's mean value theorem gives a point with . Find , correct to two decimal places.

Answer and solution

Answer: (accepted range )

  1. , so the average slope is .

  2. , so .

  3. , which lies in (the negative root does not).

OFFICIAL GATE 2027 SYLLABUS WORDING

Copied from the GATE 2027 syllabus; see all 2027 syllabi.

GATE DA

Functions of a single variable, limit, continuity and differentiability

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