# Writing a complex number in a+bi form

• February 19th 2013, 07:13 AM
Egoyan
Writing a complex number in a+bi form
Hi guys,

I'm asked to write the number 5/i in the a+bi form. The format is such that I can only enter the values of a and b in the computer. I'm not sure what I should write. Can anyone help me please?

Thanks a lot!

Egoyan
• February 19th 2013, 07:25 AM
Plato
Re: Writing a complex number in a+bi form
Quote:

Originally Posted by Egoyan
Hi guys,

I'm asked to write the number 5/i in the a+bi form. The format is such that I can only enter the values of a and b in the computer. I'm not sure what I should write. Can anyone help me please?

Learn this fact, it will save your life in complex numbers.
$(\forall z\in\mathbb{C}\setminus\{0\})\left[\frac{1}{z}=\frac{\overline{z}}{|z|^2}\right]$.

So $\frac{1}{-3+4i}=\frac{-3-4i}{5}$.

This $\frac{5}{i}=\frac{-5i}{1}=-5i$.
• February 19th 2013, 07:28 AM
Egoyan
Re: Writing a complex number in a+bi form
Oh my. I did not know that. Heh. Thanks a lot! I won't forget it any time soon now...
• February 19th 2013, 07:37 AM
HallsofIvy
Re: Writing a complex number in a+bi form
More generally, a complex number, a+ bi, multiplied by its "complex conjugate", a- bi, gives a non-negative real number: $(a+ bi)(a- bi)= a^2+ abi - abi- b^2i^2= a^2+ b^2= |a|$. In particular, you can make denominator or a fraction a real number by multiplying the numerator and denominator by the complex conjugate of the denominator:
$\frac{a+ bi}{c+ di}= \frac{a+ bi}{c+ di}\frac{c- di}{c- di}= \frac{(ac+ bd)+ i(bc- ad)}{c^2+ d^2}= \frac{ac+ bd}{c^2+ d^2}+ \frac{bc- ad}{c^2+ d^2}i$.