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CLASS_11_MATHS_SOLUTIONS_NCERT

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Class XI Chapter 5 – Complex Numbers and Quadratic Equations Maths<br />

______________________________________________________________________________<br />

Question 14:<br />

Find the real numbers x and y if x iy35i<br />

is the conjugate of 6 24i .<br />

Solution 14:<br />

Let z x iy3<br />

5i<br />

<br />

<br />

2<br />

z 3x 5xi 3yi 5yi 3x 5xi 3yi 5y 3x 5y i 5x 3y<br />

3 5 5 3 <br />

z x y i x y<br />

It is given that, z 6 24i<br />

<br />

3x 5y i 5x 3y 6 24i<br />

Equating real and imaginary parts, we obtain<br />

<br />

<br />

3x 5y 6 ........ i<br />

5x 3y 24 ........ ii<br />

Multiplying equation (i) by 3 and equation (ii) by 5 and then adding them, we obtain<br />

9x15y 18<br />

25x15y<br />

120<br />

34x<br />

102<br />

102<br />

x 3<br />

34<br />

Putting the value of x in equation (i), we obtain<br />

<br />

3 3 5y<br />

6<br />

5y<br />

6 9 15<br />

y 3<br />

Thus, the values of x and y are 3 and<br />

3 respectively.<br />

Question 15:<br />

Find the modulus of 1 i<br />

1 i<br />

<br />

1i<br />

1 i<br />

.<br />

Solution 15:<br />

1 i 1<br />

i<br />

<br />

2 2<br />

1i<br />

1i<br />

<br />

1 i 1 i 1 i 1<br />

i<br />

<br />

2 2<br />

1 2 1 2<br />

i i i i<br />

2 2<br />

1 1

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