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All questions of Differentiation- Continuity and Differentiability for A Level Exam

The area of triangle whose adjacent sides are is :
  • a)
    √70/2 sq. units
  • b)
    9√2 /2 sq. units
  • c)
    3√3 /2 sq. units
  • d)
    2√3 /2 sq. units
Correct answer is option 'A'. Can you explain this answer?

Suresh Iyer answered
Area of triangle = ½(a * b)
a = (1, 0, -2)   b = (2, 3, 1)
= i(0 + 6) + j(-4 - 1) + k(3 - 0)
= 6i - 5j + 3k
|a * b| = (36 + 25 + 9)½
|a * b| = (70)½
Area of triangle = ½(a * b)
= [(70)½]/2

The function f(x) = ax, 0 < a < 1 is​
  • a)
    increasing
  • b)
    strictly decreasing on R
  • c)
    neither increasing or decreasing
  • d)
    decreasing
Correct answer is option 'D'. Can you explain this answer?

Krishna Iyer answered
 f(x) = ax
Taking log bth the sides, log f(x) = xloga
f’(x)/ax = loga
f’(x) = ax loga   {ax > 0 for all x implies R, 
for loga e<a<1 that implies loga < 0}
Therefore, f’(x) < 0, for all x implies R
f(x) is a decreasing function.

Using approximation find the value of 
  • a)
    2.025
  • b)
    2.001
  • c)
    2.01
  • d)
    2.0025
Correct answer is option 'D'. Can you explain this answer?

Gunjan Lakhani answered
Let x=4, Δx=0.01
y=x^½ = 2
y+Δy = (x+ Δx)^½ = (4.01)^½
Δy = (dy/dx) * Δx
Δy = (x^(-1/2))/2 * Δx
Δy = (½)*(½) * 0.01
Δy = 0.25 * 0.01
Δy = 0.0025
So, (4.01)^½ = 2 + 0.0025 = 2.0025

  • a)
    cos(sin-1x) + c
  • b)
    sin-1x + c
  • c)
    sin(cos-1x) + c
  • d)
    x + c
Correct answer is option 'D'. Can you explain this answer?

Divey Sethi answered
cos(sin-1x)/(1-x2)½……………….(1)
t = sin-1 x
dt = dx/(1-x2)½
Put the value of dt in eq(1)
= ∫cost dt
= sint + c
= sin(sin-1 x) + c
⇒ x + c

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

A vector of magnitude 14 units, which is parallel to the vector
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'C'. Can you explain this answer?

EduRev JEE answered
Given vector = i + 2j - 3k
Magnitude = √12 + 22 + (-3)2 = √14
Unit vector in direction of resultant = (i + 2j - 3k) / √14
Vector of magnitude 14​ unit in direction of resultant,
⇒ 14[ (i + 2j - 3k) / √14 ]
⇒ √14(i + 2j - 3k)

  • 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

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.
 

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)

The maximum value of f (x) = sin x in the interval [π,2π] is​
a) 6
b) 0
c) -2
d) -4
Correct answer is option 'B'. Can you explain this answer?

Kiran Mehta answered
f(x) = sin x
f’(x) =cosx 
f”(x) = -sin x
f”(3pi/2) = -sin(3pi/2)
= -(-1)
=> 1 > 0 (local minima)
f(pi) = sin(pi) = 0
f(2pi) = sin(2pi) = 0 
Hence, 0 is the maxima.

Evaluate:  
  • a)
  • b)
    1/√3 arc tan[(x-2)/√5] + C
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Deepak Kapoor answered
  
Let's apply the integral substitution,
substitute 
Now use the standard integral :
substitute back u=(x-2) and add a constant C to the solution,
 

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

Leelu Bhai answered
I = ∫√(x² + 5x)dx= ∫√(x² + 5x + 25/4 - 25/4)= ∫√{(x + 5/2)² - (5/2)²}={1/2(x+5/2)(√x² + 5x)} - {25/8 log{(x + 5/2)+√x²+ 5x}}= {(2x + 5)/4 (√x² + 5x)} - {25/8 log{(x + 5/2)+√x²+ 5x}}Thus, option D is correct...

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).

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.

If  and , then the value of scalars x and y are:
  • a)
    x = 1 and y = -2
  • b)
    x = -2 and y = 1
  • c)
    x = 2 and y = -1
  • d)
    x = 2 and y = 1
Correct answer is option 'C'. Can you explain this answer?

Sushil Kumar answered
Given, a = i + 2j
b = -2i + j
c = 4i +3j
Also, c = xa +yb
Now putting the values in above equation,
4i + 3j  = x(i + 2j) + y(-2i +j)
⇒ xi + 2xj - 2yi + yj
⇒ (x-2y)i + (2x+y)j
We get,
x - 2y = 4
2x + y = 3
After solving,            
x = 2
y = -1

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

Dr Manju Sen 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

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

The unit vector in the direction of , where A and B are the points (2, – 3, 7) and (1, 3, – 4) is:
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'A'. Can you explain this answer?

Sushil Kumar answered
Given, Point A (2,-3,7)
Point B (1,3,-4)
Let vector in the direction of AB be C.
∴ C = B - A
⇒ (1,3,-4) - (2,-3,7)
⇒ ( 1-2 , 3+3 , -4-7 )
⇒ (-1,6,-11)
⇒ -1i + 6j -11k
Magnitude of vector C
|C| = √(-1)2 + 62 + (-11)2
⇒ √1+36+121
⇒ √158
Unit vector = (Vector)/(Magnitude of vector)
Unit vector C = (C vector)/(Magnitude of C vector)  = (-1i + 6j -11k)/√158

The points with position vectors  are collinear vectors, Value of a =​
  • a)
    -20
  • b)
    20
  • c)
    -40
  • d)
    40
Correct answer is option 'C'. Can you explain this answer?

Om Desai answered
Position vector A = 60i+3j
Position vector B = 40i-8j
Position vector C = aj-52j
Now, find vector AB and BC
AB = -20i-11j
BC= (a-40)i-44j
To be collinear,  angle between the vector AB and BC made by the given position vectors should be 0 or 180 degree.
That’s why the cross product of  the vectors should be zero
ABXBC=(-20i-11j)X(a-40)i-44j
0i+0j+(880+11(a-40))=0
a-40= -80
a=-40
Therefore, a should be -40 to be the given positions vectors collinear.

The value of  is:
  • a)
    0
  • b)
    3
  • c)
    1/3
  • d)
    1
Correct answer is option 'B'. Can you explain this answer?

Sahil Soni answered
Cross multiply in maths take place in cycle like i》j》k i×j=k j×k=i k×i=j but j×i=-k k×j=-i i×k=-j and dotmultiply takes place as i.i=1j.j=1K.K=1BUTI.J=0J.K=0k.i=0so the correct answer is b

 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

 Find the value of x when x is a natural number and 24x< 100.
  • a)
    {5,6,……..∞}
  • b)
    {1,2,3,4}
  • c)
    {1,2,3,4,5}
  • d)
    {0,1,2,3,4}
Correct answer is option 'B'. Can you explain this answer?

Shreya Gupta answered
We are given: 24x < 100
24x < 100
=> 24x /24 < 100/24 [Dividing both sides by positive number.]
=> x < 25/6

When x is a natural number, in this case, the following values of x make the statement true

x = 1, 2, 3, 4.

The solution set of the inequality is {1, 2, 3, 4}.

Pairs of consecutive even positive integers, both of which are larger than 7 such that their sum is less than 28, are
  • a)
    (8, 9), (9, 10), (10, 11)
  • b)
    (8, 10), (10, 12)
  • c)
    (8, 10), (10, 12), (12, 14)
  • d)
    (8, 10), (10, 12), (12, 14), (14, 16)
Correct answer is option 'C'. Can you explain this answer?

Neha Joshi answered
Let x be the smaller of the two consecutive even positive integers .
Then the other integer is x+2.
Since both the integers are larger than 7,x > 7 ....(1)
Also the sum of the two integers is less than 28.
x + (x + 2) < 28
⇒ 2x + 2 < 28
⇒ 2x < 28 − 2
⇒ 2x < 26
=> x < 13…...(2)
From(1) and (2), we get
7 < x <13
Since x is an even number, x can take the values 8, 10 and 12.
Thus the required possible pairs are (8,10),(10,12) and (12,8).

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