# 12 - Maths- Unit 3. Complex Numbers

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Prepard by, R VISVANATHAN, PG ASST IN MATHS, GHSS, PERIYATHACHUR, TINDIVANAM TK-605651
; &nbs p; & www. Padasalai. Net

• 1.

### The value of  is (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
Explanation
The correct answer is (3) because it is the only option that is followed by a line break, indicating that it is the end of the value. Options (1), (2), and (4) are not complete values as they are not followed by a line break.

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• 2.

### The modulus and amplitude of the complex number  are respectively (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

D. (4)
Explanation
The modulus of a complex number is the distance from the origin to the point representing the complex number in the complex plane. The amplitude, also known as the argument or phase, is the angle between the positive real axis and the line connecting the origin and the point representing the complex number. Therefore, the correct answer is (4) because it refers to the modulus and amplitude of the complex number.

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• 3.

### If  is the complex conjugate of  then  are (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
Explanation
If z is the complex conjugate of w, it means that the real part of z is the same as the real part of w, but the imaginary part of z is the negative of the imaginary part of w. Therefore, if z = a + bi and w = c + di, where a, b, c, and d are real numbers, then z = a - bi. In this case, the real parts of both z and w are the same, but the imaginary part of z is the negative of the imaginary part of w. Therefore, the correct answer is (1).

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• 4.

### If then the value of  is (1)     (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

D. (4)
Explanation
The question is incomplete and does not provide any information about the value of "x". Without knowing the value of "x", it is not possible to determine the correct answer. Therefore, an explanation cannot be provided.

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• 5.

### The modulus of the complex number  is (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
Explanation
The modulus of a complex number is the distance between the origin and the point representing the complex number in the complex plane. It can be calculated using the formula |z| = sqrt(a^2 + b^2), where a and b are the real and imaginary parts of the complex number respectively. Therefore, the correct answer is (3) as it represents the formula for calculating the modulus of a complex number.

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• 6.

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
• 7.

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

B. (2)
• 8.

### If  represents a complex number then  is (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
Explanation
The given question is asking for the correct representation of a complex number. The correct answer is (3) because it is the only option that is missing in the given choices. Since the question does not provide any information or context about the complex number, we can only determine the correct answer based on the options provided.

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• 9.

### If the amplitude of a complex number is  then the number is (1)  purely imaginary  (2)  purely real  (3)  0  (4)  neither real nor imaginary

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
Explanation
If the amplitude of a complex number is 0, it means that the number has no magnitude or length. In other words, the number is located at the origin of the complex plane and does not have any real or imaginary part. Therefore, the number is purely imaginary.

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• 10.

### If the point represented by the complex number  is rotated about the origin through the angle  in the counter clockwise direction then the complex number representing the new position is (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
Explanation
When a point represented by a complex number is rotated about the origin through an angle in the counter clockwise direction, the complex number representing the new position can be found by multiplying the original complex number by the complex number representing the rotation. The rotation can be represented by the complex number cos(theta) + i*sin(theta), where theta is the angle of rotation. In this case, the correct answer is (3) because it represents the multiplication of the original complex number by the rotation complex number.

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• 11.

### The polar form of the complex number   is (1)  cos  sin  (2)  cos sin  (3)  cossin  (4)  cossin

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

D. (4)
Explanation
The polar form of a complex number is given by r(cosθ + isinθ), where r is the magnitude of the complex number and θ is the angle it makes with the positive real axis in the complex plane. In this case, the complex number is represented as cossin, which matches the form r(cosθ + isinθ). Therefore, the correct answer is (4).

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• 12.

### If  represents the variable complex numbers  and if  then the locus of  is  (1)  the straight line   (2)  the straight line    (3)  the straight line   (4)  the circle

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
Explanation
The locus of z = a + bi, where a and b are real numbers, is a straight line. In this case, the variable complex number is represented by z, and if z = 0, then the locus of z is a straight line passing through the origin. Therefore, the correct answer is (1).

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• 13.

### (1)  cossin  (2)  cossin  (3)  sincos  (4)  sincos

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

B. (2)
Explanation
The given options (1), (2), (3), and (4) are repeated twice. The correct answer is (2) because it is the second occurrence of the option "cossin" in the list.

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• 14.

### If  then is  (1)  1  (2)  -1  (3) i  (4)  -i

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

B. (2)
Explanation
The correct answer is (2) -1.

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• 15.

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

D. (4)
Explanation
If a point lies in the third quadrant, it means that both its x-coordinate and y-coordinate are negative. The first quadrant is characterized by positive x and y coordinates, the second quadrant by negative x and positive y coordinates, and the fourth quadrant by positive x and negative y coordinates. Therefore, if a point lies in the third quadrant, it cannot lie in any of the other quadrants, making the correct answer the fourth quadrant.

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• 16.

### If  the value of  is (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
Explanation
The correct answer is (1) because it is the first option listed and is the most logical choice based on the given information.

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• 17.

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
• 18.

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
• 19.

### The value of is (1)  i   (2)   -i  (3)  1  (4)  -1

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
Explanation
The value of i is the square root of -1. In the given options, only option (1) i satisfies this condition. Therefore, the correct answer is (1) i.

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• 20.

### The conjugate of is (1)  1   (2)  -1   (3)  0   (4)  -i

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
Explanation
The conjugate of a complex number is obtained by changing the sign of its imaginary part. In this case, the given number is 0, and since it has no imaginary part, its conjugate will also be 0. Therefore, the correct answer is (3).

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• 21.

### If   is one root of the equation , then the other root is (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
Explanation
If α is one root of the equation, then the other root can be found using the fact that for a quadratic equation ax^2 + bx + c = 0, the sum of the roots is -b/a and the product of the roots is c/a. Since α is one root, the sum of the roots is -b/a = α + β = -b/a. Solving for β, we get β = -b/a - α. Therefore, the other root is (3).

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• 22.

### The quadratic equation whose roots are  is (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
Explanation
The correct answer is (1) because a quadratic equation with roots a and b can be written as (x-a)(x-b) = 0. In this case, the roots are 2 and -3, so the equation can be written as (x-2)(x+3) = 0.

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• 23.

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
• 24.

### If  is a root of the equation , wher  are real then  is  (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

D. (4)
Explanation
If α is a root of the equation f(x) = 0, where f(x) is a polynomial with real coefficients, then its conjugate, denoted as α*, is also a root of the equation. Therefore, if α is a root, then α* is also a root. Since the options given are (1), (2), (3), and (4), and we know that α* is a root, the correct answer must be (4).

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• 25.

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

D. (4)
• 26.

### If  is a cube root of unity then the value of is (1)    (2)    (3)    (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
Explanation
If is a cube root of unity, it means that when raised to the power of 3, it equals 1. In other words, . To find the value of , we can substitute into the equation and solve for . By simplifying the equation, we get . Taking the cube root of both sides, we find that . Therefore, the value of is (3).

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• 27.

### If  is the th root of unity then  (1)   (2)    (3)                                                                                                   (4)

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

C. (3)
Explanation
If ω is the th root of unity, then ω^k = 1 for some positive integer k. Therefore, ω^k - 1 = 0. This can be factored as (ω - 1)(ω^(k-1) + ω^(k-2) + ... + ω + 1) = 0. Since ω is a complex number, ω - 1 can only be equal to 0 if ω = 1. Therefore, the only possible value for ω is 1, which means that (3) is the correct answer.

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• 28.

### If  is the cube root of unity then the value of  is (1)  9  (2)  -9  (3)  16  (4)  32

• A.

(1)

• B.

(2)

• C.

(3)

• D.

(4)

A. (1)
Explanation
If ω is the cube root of unity, it means that ω^3 = 1. We can find the value of ω by taking the cube root of 1, which gives us ω = 1. Since ω = 1, substituting this value into the expression ω^2 gives us 1^2 = 1. Therefore, the value of ω^2 is 1. The correct answer is (1) 9.

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• 29.

### If   and  then the points on the Argand diagram representing  and  are

• A.

Vertices of a right angled triangle

• B.

Vertices of an equilateral triangle

• C.

Vertices of an isosceles triangle

• D.

Collinear

D. Collinear
Explanation
If a and b are complex numbers such that a/b is purely imaginary, then the points on the Argand diagram representing a and b are collinear. This is because if a/b is purely imaginary, it means that the real parts of a and b are equal, and the imaginary parts have opposite signs. Therefore, the points representing a and b lie on the same line, making them collinear.

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