# Complex Number Notes for H2 A Level Math

## Imaginary number i

i is an imaginary number, where i = √(-1).

Since i = √(-1), i² = -1.

## Ways to represent complex numbers

Complex numbers consist of both the imaginary and the real portion. We can represent complex numbers in :

• cartesian from i.e. x + iy
• exponential form:
• trigonometric form i.e. r(cosθ + i sinθ)

## Representing a complex number on the argand diagram

For a complex number, in cartesian from e.g. z = x + iy, x refers to its x coordinate, and y refers to its y coordinate.

For a complex number in exponential or trigonometric form, we use r and θ to locate the complex number on the argand diagram. r and θ refers to:

• r is the length between the origin and the complex number on the argand diagram
• θ is the angle the complex number makes with the positive x- axis (or real axis)

## Modulus Argument Form

For a complex number z, it can be written as z = x + iy

|z| = r= √(x² +y²)

θ = arg(z) = the angle z makes with the positive x-axis in the argand diagram, where -𝞹<arg(z)≤𝞹

## Some Useful Results

|z₁z₂| = |z₁||z₂|

arg(z₁z₂) = arg(z₁)+arg(z₂)

|z₁/z₂| = |z₁|/|z₂|

arg(z₁/z₂) = arg(z₁)-arg(z₂)

|z₁ⁿ| =|z₁|ⁿ

arg(z₁ⁿ) = n arg(z₁)

## Conjugate

We use the symbol * to denote conjugate of a complex. For instance, the conjugate of the complex number z, can be written as z*.

### Finding conjugate of a complex number in cartesian form

If z = x + iy, then its conjugate, z* = x – iy.

### Finding conjugate of a complex number in trigonometric or exponential form

If |z| = r, and arg(z) = θ, then for the conjugate of z, i.e. z*:

|z*| = r

arg(z*) = -θ

## Solving Equations involving complex roots

If all the coefficients are real, then the complex roots occur in conjugate pairs.

## H2 A Level Math Notes

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