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Maths > Complex Numbers and Quadratic Equations > Basics of Complex Numbers
Complex Numbers and Quadratic Equations

Basics of Complex Numbers

Do all the equations have a solution? What if the solution of an equation does not lie on the number line? What is the square root of a negative number? These questions led us to the discovery of a new set of numbers known as the Complex numbers or the imaginary numbers. Let us find out more!

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Let us try and solve the equation x2 + 1 = 0. We  can simplify it and write it as x2 = -1 or x = ± \( \sqrt[]{-1} \) . But what is the root of -1? It is not on the number line. So does it simply not exist? Well in mathematics, sticking to reality has never been a priority! This solution does not lie on the number line, that means it must be off it then.

We can write -a = (-1)×(a), where ‘a’ is a real number. The square root of a negative number can be written as \( \sqrt[]{(-1)(a)} \) = [\( \sqrt[]{-1} \)][ \( \sqrt[]{a} \) ]

Let us use a symbol for \( \sqrt[]{-1} \) . Let’s denote it by ‘i’ ( from iota, Greek for extremely small). Then i = \( \sqrt[]{-1} \) is a number that doesn’t fall anywhere on the number line! Such numbers that are not on the real number line, are the imaginary numbers. They are also known as the complex numbers.

Square Root of a Negative Number

Complex numbers are used in the development of many scientific theories

“Now wait for a second, we only have ‘i’, why are you calling it numbers? Where are the others?” We can get every real number from other real numbers by certain algebraic operations. Like if you keep on adding the number one to itself, you will get the set of natural numbers and so on.

Browse more Topics under Complex Numbers And Quadratic Equations

Similarly, all the numbers that have ‘i’ in them are the imaginary numbers. The number i, is the imaginary unit. 3i, 4i, -i, \( \sqrt[]{-9} \) etc. are all imaginary numbers. Let us have a general definition of the imaginary or complex numbers.

Why So Complex!

Let ‘a’ , ‘b’ be two real numbers. Let i = \( \sqrt[]{-1} \), then any number of the form a + ib is a complex number. The reason to define a complex number in this way is to make a connection between the real numbers and the complex ones. For example, we can write, 2 = 2 + 0.i.  Therefore, every real number can be written in the form of a + ib; where b = 0.

Also if a complex number is such that a = 0, we call it a purely imaginary number. In general, a is known as the “real” part and b is known as the “imaginary” or the complex part of the imaginary number. “Why am I doing this again?” Well, hold on. Let’s recall the equation that we couldn’t quite figure out the solution for i.e. x2 + 1 = 0

Hence, x = ± \( \sqrt[]{-1} \) or x = ± i. Substituting for x, in the equation we have (i)2 + 1 = 0

i2 = ± [ \( \sqrt[]{-1} \) ]2 = -1, using the above equation we have, (-1) + 1 = 0 satisfies the equation. Substitution of -i also gives the same results. So there you go, now you can solve equations that you would have rather just left alone. To sum up, all the numbers of the form a +ib, where a and b are real and i = \( \sqrt[]{-1} \) , are called imaginary numbers. We usually denote an imaginary number by ‘z’.

Equality of Complex Numbers

Let us practice the concepts we have read this far.

Example 1: There are two numbers z1 = x + iy and z2 = 3 – i7. Find the value of x and y for z1 = z2. What is the sum of Re (z1, z2)?

Solution: We have z1 = x + iy and z2 = 3 – i7

First of all, real part of any complex number (a+ib) is represented as Re(a + ib) = a and imaginary part of (a +ib) is represented as Im(a+ib) = b. Also, when two complex numbers are equal, their corresponding real parts and imaginary parts must be equal. Therefore, if a + ib = c + id, then Re(a+ib) = Re(c+id) and Im(a+ib) = Im(c+id)

Conversely if two complex numbers say z1 and z2, are such that Re(z1) = Re(z2) and Im(z1) = Im(z2), then z1=z2. In other words, we must have a = c and b = d. We can’t have a = d because we can’t relate the real and the imaginary parts together. Let us come back to our problem.

z1 =  z2 or x + iy = 3 – i7

Re(x + iy) = x                 Re(3 – i7) = 3

Im(x + iy) = y                 Im(3 – i7) = -7

This means x = 3 and y = -7. Therefore, Re(z1,z2) = 3,3 and hence, the sum of real parts of z1 and z2 = 3 + 3 = 6.

Integer Powers of  ‘i’

As we saw already, i2 = -1. Also, i3 = i2(i) = -i. Similarly, you can find any given power of i, by reducing it to the above two forms.

Learn the Operations on Complex Numbers here.

More Solved Examples For You

Question 2: If 2i2+6i3+3i166i19+4i25  x+iy, then (x,y)= ?

A) (1 , 4)                                  B) (4 , 1)

C) (-1 , 4)                                 D) (-1 , -4)

Answer : A) The above expression can be simplified as:

2 (-1) + 6 (-i) + 3 ( i )2×2×2×2 – 6 (i18 . i) + 4 (i24  .  i) = x + iy

Therefore, -2 – 6i + 3 (1) – 6 (-1)9.i +  4i = x + iy Or 1 + 4i = x + iy

Therefore, x = 1 and y = 4 or (x,y) = (1,4)

Question 3: Is a complex number real a real concept?

Answer: Basically, a complex number consists of two parts. First, a real number and second an imaginary number. In addition, these numbers are the building blocks of more complex math, such as algebra. Moreover, we can apply them to various aspects of real-life such as electromagnetism and electronics.

Question 4: State the way to convert complex numbers in standard form?

Answer: Firstly, multiply then in the same manner as polynomials. After that, simplify the expression and write the answer in standard form. By following this process you can write a complex number in standard form.

Question 5: What is a pure imaginary number?

Answer: A number is said to be purely imaginary if it has no real part that is the term is often used in fondness to the simpler “imaginary” situation. Here it assumes complex values with non-zero real parts, however, in some cases the real part is identically zero.

Question 6: What are the complex numbers in standard form?

Answer: In standard form, the complex number refers to the number which can be written as a + bi where a and b are real number and they can be positive, negative, fraction, decimal, integers, zero, etc.

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