Homework Help: Working complex fractions without conjugate method

1. Aug 11, 2011

LF07LAN

1. The problem statement, all variables and given/known data
Ive been given a question that requires an answer in polar form but the method I must use is the normal addition/subtraction of fractions. This throws me because i'm sure there is a simple method for the reciprocal of a complex number.

Q. 1/(13- 5i) - 1/(2-3i)

2. Relevant equations

where z = a + bi
i'm aware of how to change to polar for whole numbers and how to get where I want using the conjugate method but cant fathom the correct start point.

3. The attempt at a solution

I considered 1/13 - 1/5i using the rules of fractions but wasn't convinced.
The alternative was to convert straight to polar using 1 / 0degrees over the (13-5i in polar form)

Last edited: Aug 11, 2011
2. Aug 11, 2011

HallsofIvy

$$\frac{1}{a+ bi}= \frac{1}{a+ bi}\frac{a- bi}{a- bi}= \frac{a- bi}{(a+ bi)(a- bi)}$$
$$= \frac{a- bi}{a^2-b^2i^2}= \frac{a- bi}{a^2+ b^2}$$

(Even for real numbers,
$$\frac{1}{a+ b}\ne \frac{1}{a}+ \frac{1}{b}$$)

3. Aug 11, 2011

LF07LAN

Doesn't that just proove the Complex Conjugate method? I can only solve the answer without the use of this method.

The actual wording of the question is:
Evaluate the following expression using normal addition/subtraction of fractions and/or polar form (i.e DO NOT use the complex conjugate method) giving the result in polar form.

if I use (a-bi)/(a2 + b2) doesnt this just mean I've used the complex Conjugate Method? Or ar you saying I can do it this way:

where a^2 + b^2 = 13^2 + 5^2 = 194

13/194 + (5/194)i

0.067 + 0.026i
and the same for the second rectangular element of the question (1/(2-3i)

4. Aug 11, 2011

Redbelly98

Staff Emeritus
Welcome to PF.

I think "normal addition/subtraction of fractions" means you find a common denominator for the two fractions, and then you can add the resulting numerators.