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How do I multiply complex numbers?

A complex number has the form a+bi. We call a the 'real' bit (ie. the bit on a regular number line) and b is the 'imaginary bit.

Multiplying complex numbers is done in a very similar way to multiplying out brackets. However, you need to remember that i2 = -1.

For example: what is (6+i) x (5+2i)?
We multiply out the brackets, and get: 6x5 + 6x2i + ix5 +ix2i
This gives 30 + 12i + 5i - 2 (because i2=-1).
Collecting like terms we get 28+17i which is our answer.

3 years ago

Answered by Chloe, an A Level Maths tutor with MyTutor

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