Step by step
The question
Question: work out 10/(3 − i).
Remember: i is the square root of minus 1.
So i squared is minus 1.
1 Multiply by the conjugate
The bottom has an i in it.
To remove it, we use the conjugate of 3 − i: the same number with the sign of the i part changed.
It is 3 + i.
We multiply the top and the bottom by 3 + i.
(3 + i)/(3 + i) is 1, so the value stays the same.
2 Multiply out
On top, times each part.
On the bottom too.
3 Change i² to −1
i² is minus 1.
So −i² change sign.
On the bottom, 3i − 3i make zero.
Cross them out.
That is why we use the conjugate.
4 Group like parts
Real parts together, i parts together.
Top: 30 + 10i.
Bottom: 10, a plain number.
No i at the bottom now.
5 Split the fraction
Each part on top is divided by 10.
30/10 is 3.
10/10 is 1.
So the answer is 3 + i.
6 Check
Let's check: times 3 − i undoes divided by 3 − i.
Our top 30 + 10i times 3 − i is 100.
That is 10 times the question's top, 10, and the bottom is 10.
So the answer is right!
Let's look back.
First: multiply by the conjugate.
Then: multiply out.
Then: change i² to −1.
Then: group like parts.
Then: split the fraction.
Last: check.