Trigonometry MCQ Quiz for TGT/PGT Maths Practice Test with Timer
Comprehensive Q&A Collection
31. If f(x) = x – [x], where [x] is the greatest integer less than or equal to x, then f(-1/2) is equal to:
Correct Answer: (d) 1/2
Explanation:
The function f(x) = x – [x] represents the fractional part of x.
We need to find f(-1/2). First, we find the value of [-1/2].
The greatest integer less than or equal to -0.5 is -1. So, [-1/2] = -1.
Now, substitute this value into the function:
f(-1/2) = -1/2 – [-1/2] = -1/2 – (-1) = -1/2 + 1 = 1/2.
S.C.R.A. 2000
33. The domain of f(x) = 1 / √(|x| – x) is:
Correct Answer: (a) (-∞, 0)
Explanation:
For the function to be defined, the expression inside the square root must be strictly positive (since it’s in the denominator).
So, |x| – x > 0, which means |x| > x.
We analyze this condition in two cases:
- If x ≥ 0, then |x| = x. The inequality becomes x > x, which is false for all x.
- If x < 0, then |x| = -x. The inequality becomes -x > x. Adding x to both sides gives 0 > 2x, or x < 0.
The condition |x| > x is only true when x is a negative number. Therefore, the domain is x < 0, or (-∞, 0).
Raj. (P.E.T.) 2000; Kerala (C.E.E.) 1989
39. The function sin(x/3) is periodic with period:
Correct Answer: (b) 6π
Explanation:
The period of the standard sine function, sin(x), is 2π. For a function of the form f(x) = sin(bx), the period is given by the formula T = 2π / |b|.
In this case, the function is sin(x/3), so b = 1/3.
Therefore, the period T = 2π / (1/3) = 2π * 3 = 6π.
Pb. (C.E.T.) 1999
47. If f(x) = cos(log x), then f(x)f(y) – 1/2[f(x/y) + f(xy)] has the value:
Correct Answer: (d) 0
Explanation:
We are given f(x) = cos(log x). Let’s evaluate each term in the expression.
- f(x)f(y) = cos(log x)cos(log y)
- f(x/y) = cos(log(x/y)) = cos(log x – log y)
- f(xy) = cos(log(xy)) = cos(log x + log y)
Now substitute these into the main expression:
Expression = cos(log x)cos(log y) – 1/2[cos(log x – log y) + cos(log x + log y)]
We use the trigonometric identity: cos(A – B) + cos(A + B) = 2cosAcosB.
Let A = log x and B = log y. Then the expression in the bracket becomes:
[cos(log x – log y) + cos(log x + log y)] = 2cos(log x)cos(log y).
Substituting this back:
Expression = cos(log x)cos(log y) – 1/2[2cos(log x)cos(log y)]
Expression = cos(log x)cos(log y) – cos(log x)cos(log y) = 0.
Karnataka (C.E.T.) 1999; Kerala C.E.E. 2001
52. Let f(x) = 4x / (4x + 2). Then f(x) + f(1 – x) is equal to:
Correct Answer: (b) 1
Explanation:
We are given the function f(x) = 4x / (4x + 2).
First, let’s find the expression for f(1 – x):
f(1 – x) = 4(1-x) / (4(1-x) + 2) = (4/4x) / ((4/4x) + 2)
To simplify, multiply the numerator and denominator by 4x:
f(1 – x) = (4) / (4 + 2 * 4x) = 4 / (2(2 + 4x)) = 2 / (4x + 2)
Now, let’s add f(x) and f(1 – x):
f(x) + f(1 – x) = [4x / (4x + 2)] + [2 / (4x + 2)]
Since they have a common denominator, we can add the numerators:
f(x) + f(1 – x) = (4x + 2) / (4x + 2) = 1.
Roorkee 1998
61. If f(x) = (a – xn)1/n, then f(f(x)) equals:
Correct Answer: (a) x
Explanation:
To find f(f(x)), we substitute the expression for f(x) into the function itself.
f(f(x)) = f( (a – xn)1/n )
Now, replace ‘x’ in the original function definition with ‘(a – xn)1/n‘:
f(f(x)) = [a – { (a – xn)1/n }n ]1/n
The powers 1/n and n cancel each other out:
f(f(x)) = [a – (a – xn)]1/n
Simplify the expression inside the brackets:
f(f(x)) = [a – a + xn]1/n = [xn]1/n
Again, the powers n and 1/n cancel out, leaving:
f(f(x)) = x.
C.E.T. 1997; Roorkee 1997
69. The minimum value of sin6x + cos6x is:
Correct Answer: (a) 1/4
Explanation:
We can simplify the expression f(x) = sin6x + cos6x.
- Rewrite using a3 + b3 identity, where a = sin2x and b = cos2x: f(x) = (sin2x)3 + (cos2x)3
- Using a3 + b3 = (a + b)3 – 3ab(a + b): f(x) = (sin2x + cos2x)3 – 3(sin2x)(cos2x)(sin2x + cos2x)
- Since sin2x + cos2x = 1: f(x) = (1)3 – 3sin2x cos2x(1) = 1 – 3sin2x cos2x
- Use the double angle identity sin(2x) = 2sinxcosx, so sin2x cos2x = (1/4)sin2(2x): f(x) = 1 – 3 * (1/4)sin2(2x) = 1 – (3/4)sin2(2x)
- To find the minimum value of f(x), we must subtract the maximum possible value of (3/4)sin2(2x). The maximum value of sin2(2x) is 1. Minimum value of f(x) = 1 – (3/4) * 1 = 1/4.
I.I.T. 1992; C.E.T. 1996
82. If f(x) is an odd periodic function with period 2, then f(4) equals:
Correct Answer: (a) 0
Explanation:
- Periodicity: The function has a period of 2. This means f(x + 2) = f(x) for all x. We can use this to relate f(4) to f(0): f(4) = f(2 + 2) = f(2) = f(0 + 2) = f(0). So, f(4) = f(0).
- Odd Function Property: The function is odd. This means f(-x) = -f(x) for all x. Let’s apply this property for x = 0: f(-0) = -f(0) f(0) = -f(0) 2f(0) = 0 f(0) = 0.
- Conclusion: Since f(4) = f(0) and f(0) = 0, it follows that f(4) = 0.
I.I.T. 1998
87. If the function f(x) = cos2x + cos2(π/3 + x) + cos2(π/3 – x) is constant, then the value of this constant is:
Correct Answer: There appears to be a typo in the options. The correct answer is 3/2.
Explanation:
- Use the identity cos2θ = (1 + cos(2θ))/2.
f(x) = (1 + cos(2x))/2 + (1 + cos(2(π/3 + x)))/2 + (1 + cos(2(π/3 – x)))/2 - Combine terms:
f(x) = 3/2 + 1/2 [cos(2x) + cos(2π/3 + 2x) + cos(2π/3 – 2x)] - Use the identity cos(A + B) + cos(A – B) = 2cosAcosB. Let A = 2π/3 and B = 2x.
cos(2π/3 + 2x) + cos(2π/3 – 2x) = 2cos(2π/3)cos(2x) - We know cos(2π/3) = -1/2.
So, 2(-1/2)cos(2x) = -cos(2x). - Substitute this back into the expression for f(x):
f(x) = 3/2 + 1/2 [cos(2x) – cos(2x)] = 3/2 + 1/2 [0] = 3/2.
The function is constant with a value of 3/2. Since this is not an option, there might be a typo in the question or options provided.
Roorkee 1991
95. sin-1(cos(sin-1x)) + cos-1(sin(cos-1x)) is equal to:
Correct Answer: (c) π/2
Explanation:
Let’s simplify each term separately.
- First term: sin-1(cos(sin-1x))
Let sin-1x = θ, so x = sinθ.
We need sin-1(cosθ). We know cosθ = sin(π/2 – θ).
So, sin-1(cosθ) = sin-1(sin(π/2 – θ)) = π/2 – θ = π/2 – sin-1x. - Second term: cos-1(sin(cos-1x))
Let cos-1x = φ, so x = cosφ.
We need cos-1(sinφ). We know sinφ = cos(π/2 – φ).
So, cos-1(sinφ) = cos-1(cos(π/2 – φ)) = π/2 – φ = π/2 – cos-1x. - Adding the terms:
Expression = (π/2 – sin-1x) + (π/2 – cos-1x)
= π – (sin-1x + cos-1x) - Using the identity sin-1x + cos-1x = π/2:
Expression = π – (π/2) = π/2.
Pb. (C.E.T.) 1990
101. If f(x) = (3x + 2) / (5x – 3), then:
Correct Answer: (a) f-1(x) = f(x)
Explanation:
To find the inverse function f-1(x), we follow these steps:
- Let y = f(x). So, y = (3x + 2) / (5x – 3).
- Swap x and y to start finding the inverse: x = (3y + 2) / (5y – 3).
- Solve for y:
x(5y – 3) = 3y + 2
5xy – 3x = 3y + 2
5xy – 3y = 3x + 2
y(5x – 3) = 3x + 2
y = (3x + 2) / (5x – 3). - So, f-1(x) = (3x + 2) / (5x – 3).
Comparing f-1(x) with the original function f(x), we see that they are identical. Thus, f-1(x) = f(x).
Pb. (C.E.T.) 1989
108. The domain of the function f(x) = sin-1(x – 3) / √(9 – x2) is:
Correct Answer: (c) [2, 3)
Explanation:
We must satisfy the conditions for both the numerator and the denominator.
- Numerator: sin-1(x – 3)
The argument of arcsin must be in [-1, 1].
-1 ≤ x – 3 ≤ 1
2 ≤ x ≤ 4. So, the interval is [2, 4]. - Denominator: √(9 – x2)
The expression inside the square root must be strictly positive.
9 – x2 > 0
x2 < 9
-3 < x < 3. So, the interval is (-3, 3). - Intersection: We need to find the intersection of [2, 4] and (-3, 3).
The common values are those that are greater than or equal to 2 AND less than 3.
Therefore, the domain is [2, 3).
A.I.E.E.E. 2004
111. The function f satisfies the functional equation 3f(x) + 2f((x + 59)/(x – 1)) = 10x + 30 for all x ≠ 1. The value of f(7) is:
Correct Answer: (b) 4
Explanation:
This is a system of equations problem in disguise.
- Let x = 7:
3f(7) + 2f((7 + 59)/(7 – 1)) = 10(7) + 30
3f(7) + 2f(66/6) = 70 + 30
3f(7) + 2f(11) = 100 —(Eq. 1) - Let x = 11: Notice that if we substitute x=11, the term (x+59)/(x-1) becomes (11+59)/(11-1) = 70/10 = 7. This will give us a second equation with f(7) and f(11).
3f(11) + 2f((11 + 59)/(11 – 1)) = 10(11) + 30
3f(11) + 2f(7) = 110 + 30
2f(7) + 3f(11) = 140 —(Eq. 2) - Solve the system:
Multiply Eq. 1 by 3: 9f(7) + 6f(11) = 300
Multiply Eq. 2 by 2: 4f(7) + 6f(11) = 280
Subtract the new second equation from the new first one:
(9f(7) – 4f(7)) = 300 – 280
5f(7) = 20
f(7) = 4.
Kerala (P.E.T.) 2005

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