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All questions of Continuity and Differentiability for JEE Exam

If the function f (x) = x2– 8x + 12 satisfies the condition of Rolle’s Theorem on (2, 6), find the value of c such that f ‘(c) = 0​
  • a)
    6
  • b)
    4
  • c)
    8
  • d)
    2
Correct answer is option 'B'. Can you explain this answer?

Dr Manju Sen answered
f (x) = x2 - 8x + 12
Function satisfies the condition of Rolle's theorem for (2,6).
We need to find c for which f’(c) = 0
f’(x) = 2x – 8
f’(c) = 2c – 8 = 0
c = 4

When Rolle’s Theorem is verified for f(x) on [a, b] then there exists c such that​
  • a)
    c ε [a, b] such that f'(c) = 0
  • b)
    c ε (a, b) such that f'(c) = 0
  • c)
    c ε (a, b] such that f'(c) = 0
  • d)
    c ε [a, b) such that f'(c) = 0
Correct answer is option 'B'. Can you explain this answer?

Answer is
B) c ∈ (a, b) such that f'(c) = 0.
Statement for Rolle’s Theorem :
Suppose that a function f(x) is continuous on the closed interval [a,b] and differentiable on the open interval (a,b). Then if f(a)=f(b), then there exists at least one point c in the open interval (a,b) for which f′(c)=0.
 

Differentiate sin22 + 1) with respect to θ2
  • a)
    sin(2θ2 + 1)
  • b)
    cos(2θ2 + 2)
  • c)
    sin(2θ2 + 2)
  • d)
    cos(2θ2 + 1)
Correct answer is option 'C'. Can you explain this answer?

Gunjan Lakhani answered
y = sin22+1)
v = θ2
dy/d(v) = dydθ/dvdθ
dy/dthη = sin2(V+1)
= 2sin(V+1)⋅cos(V+1)dv/dθ
= 2sin(θ2+1)cos(θ2+1)
= sin2(θ2+1).

Whta is the derivatve of y = log5 (x)
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

Tanuja Kapoor answered
y = log5 x = ln x/ln 5 → change of base 
= ln x/ln 5
dy/dx = 1/ln5⋅1/x → 1/ln5 is a constant, so we don't change it
= 1/(x ln 5)

  • a)
    – 1
  • b)
    0
  • c)
    1
  • d)
    1/2
Correct answer is option 'D'. Can you explain this answer?

Angad Gupta answered
We have to use L'Hopital Rule It is in the form 0/0 So first we have to differentiate it After differentiating we get sinx/2x Then again differentiate it We get cosx/2 and now we get the answer as 1/2

A real function f is said to be continuous if it is continuous at every point in …… .​
  • a)
    [-∞,∞]
  • b)
    The range of f
  • c)
    The domain of f
  • d)
    Any interval of real numbers
Correct answer is option 'A'. Can you explain this answer?

Its domain. This means that for any point x in the domain of f, as x approaches a certain value a, the value of f(x) approaches f(a). In other words, there are no sudden jumps or gaps in the graph of f.

More formally, a function f is continuous at a point a if:

1. f(a) is defined (i.e. a is in the domain of f).
2. The limit of f(x) as x approaches a exists (i.e. the left and right-hand limits are equal).
3. The limit of f(x) as x approaches a is equal to f(a).

If a function is continuous at every point in its domain, it is called a continuous function. Continuous functions have many useful properties and are often used in mathematical models and real-world applications.

Derivatve of f(x)   is given by
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Neha Sharma answered
 y
The derivative of y = ef(x)is dy/dx = f'(x)ef(x)
In this case, f(x) = x2 , and the derivative of x2 = 2x
Therefore, f'(x)= 2x,
 dy/dx = 2x

If 3 sin(xy) + 4 cos (xy) = 5, then   = .....
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Krishna Iyer answered
3sinxy + 4cosxy = 5
⇒ 5(3/5 sinxy + 4/5 cosxy) = 5 
⇒ (3/5 sinxy + 4/5 cosxy) = 1
now (3/5)²+(4/5)² = 1
    so let, 3/5 =   cosA
             ⇒ 4/5 = sinA
So , (3/5 sinxy + 4/5 cosxy) = 1
     ⇒ (cosAsinxy + sinAcosxy) = 1
     ⇒ sin(A+xy) = 1
     ⇒ A + xy = 2πk + π/2 (k is any integer)
     ⇒ sin⁻¹(4/5) + xy = 2πk + π/2
     differenciating both sides with respect to x
   0 + xdy/dx + y = 0
      dy/dx = -y/x

y = log(sec + tan x)
  • a)
    sec x tan x – 1
  • b)
    sec x
  • c)
    sec x tan x + 1
  • d)
    tan x
Correct answer is option 'B'. Can you explain this answer?

Lavanya Menon answered
y = log(secx + tanx)
dy/dx = 1/(secx + tanx){(secxtanx) + sec2x}
= secx(secx + tanx)/(secx + tanx)
= secx

 For what values of a and b, f is a continuous function.
  • a)
    a=2,b=0
  • b)
    a=1,b=0
  • c)
    a=0,b=2
  • d)
    a=0,b=0
Correct answer is 'A'. Can you explain this answer?

Tejas Verma answered
For continuity: LHL=RHL
at x=2,
LHL: x < 2 ⇒ f(x) = 2*a
RHL: x ≥ 2 ⇒ f(x) = 4
For continuity: LHL = RHL
⇒ 2a = 4 ⇒ a = 2
at x = 0,
LHL: x < 0 ⇒ f(x) = b
RHL: x ≥ 0 ⇒ f(x) = 0 * a
For continuity: LHL = RHL
⇒ b = 0

The value of a for which the sum of the squares of the roots of the equation x2 – (a – 2) x – a – 1 = 0 assume the least value is
  • a)
    1
  • b)
    0
  • c)
    3
  • d)
    2
Correct answer is option 'A'. Can you explain this answer?

Jaya Das answered
x2 - (a - 2) x -a-1 = 0
⇒ α + b = a- 2 ; α β = -(a+ 1)
α2 + β2 = (α + β)2 - 2αβ = a2 - 2a +6 = (a -1)2+5
For min. value of α2 + β2 where α is an integer
⇒ a = 1.

The equation 2 tan x + 5x – 2 = 0 has
  • a)
     No solution
  • b)
    At least one real solution in [0, π/4]
  • c)
     Two real solution in [0, π/4]
  • d)
    None of these
Correct answer is option 'B'. Can you explain this answer?

Krishna Iyer answered
 Let f(x) = 2tan x + 5x – 2 
Now, f(0) = – 2 < 0 
and f(π/4 ) = 2 + 5π/4 – 2 
= 5π/5 > 0, 
the line intersects the x-axis at x = ⅖ 
since 0 < ⅖ <  π/4
Thus, f(x) = 0 has atleast real solution root in ( 0, π/4)


Correct answer is option 'A'. Can you explain this answer?

Aryan Khanna answered
y = tan-1(1-cosx)/sinx
y = tan-1{2sin2(x/2)/(2sin(x/2)cos(x/2)}
y = tan-1{tan x/2}
y = x/2  => dy/dx = 1/2

Function f(x) = log x +  is continuous at​
  • a)
    (0,1)
  • b)
    [-1,1]
  • c)
    (0,∞)
  • d)
    (0,1]
Correct answer is option 'D'. Can you explain this answer?

Om Desai answered
  • [-1,1] cannot be continuous interval because log is not defined at 0.
  • The value of x cannot be greater than 1 because then the function will become complex.
  • (0,1) will not be considered because its continuous at 1 as well. Hence D is the correct option.

If f(x) = | x | ∀ x ∈ R, then
  • a)
    f is discontinuous at x = 0
  • b)
    f is derivable at x = 0 and f ‘ (0) = 1
  • c)
    f is derivable at x = 0 but f’ (0) ≠
  • d)
    none of these
Correct answer is option 'D'. Can you explain this answer?

Sakshi Jain answered
|X| is a continuous function ,which is clear from its graph but at x=0,|X| is not differentiable since it has a sharp edge at x=0 . hence 'd' is correct option.

​Find the derivate of y = sin4x + cos4x
  • a)
    – sin 2x
  • b)
    4 sin3 x + 4 cos3 x
  • c)
    – sin 4x
  • d)
    4 sin x cos x cos 2x
Correct answer is option 'C'. Can you explain this answer?

Mohit Rajpoot answered
y=sin4x,  and z=cos4x
So by using chain rule
df(x)/dx = dsin4x/dx + dcos4x/dx
=dy4/dy * dy/dx + dz4/dz * dzdx
=dy4/dy * dsinxdx + dz4/dz * dcosx/dx
=4y(4−1)⋅cosx+4z(4−1)⋅(−sinx)
=4sin3xcosx − 4cos3xsinx
=4sinxcosx(sin2x − cos2x)
=2sin2x(−cos2x)
=−2sin2xcos2x
=−sin4x

  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

Poonam Reddy answered
y + sin y = 5x
dy/dx + cos ydy/dx = 5
dy/dx = 5/(1+cos y)

Let f : R → R be a function defined by f(x) = Min {x + 1, |x| + 1}. Then which of the following is true ?
  • a)
    f(x) ³ 1 for all x Î R
  • b)
     f(x) is not differentiable at x = 1
  • c)
    f(x) is differentiable everywhere
  • d)
     f(x) is not differentiable at x = 0
Correct answer is option 'C'. Can you explain this answer?

Nikita Singh answered
f(x) = Min {x + 1,∣x∣ + 1}
x = 0 ⇒ ∣x∣ = x
f(x) = min {x + 1,x + 1}
= x + 1
x < 0,∣x∣ = −x
f(x) = min {x + 1,−x + 1}
−x + 1> x + 1
x < 0
⇒x < 0, f(x) = x + 1
x > 0,f (x) = x + 1
⇒ f(x) = x + 1 for all x
∴f(x) is differential everywhere (continuous and constant slope)

Examine the continuity of function 
  • a)
    Discontinuous at x=1,2
  • b)
    Discontinuous at x=1
  • c)
    Continuous everywhere.
  • d)
    Discontinuous at x=2
Correct answer is option 'C'. Can you explain this answer?

Om Desai answered
Lim f (x) = lim (x-1)(x-2)  at x tend to k 
► So it get    k2-3k+2
► Now f (k) = k2 -3k+2
► So f (x) =f (k) so continous at everywhere

  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Suhani Dangarh answered
Put x=tan thita. then you will get. 2 tan inverse x then differentiate

If f(x) = x + cot x, 
  • a)
    -4
  • b)
    2
  • c)
    4
  • d)
    -2
Correct answer is option 'C'. Can you explain this answer?

Aryan Khanna answered
 f(x) = x + cot x
f’(x) = 1 + (-cosec2 x)
f”(x) = 0 - 2cosec x(-cosec x cot x)
= 2 cosec2 x cot x
f”(π/4) = 2 cosec2 (π/4) cot(π/4)
= 2 [(2)^½]2 (1)
= 4

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