# quick complex number questions

• Nov 7th 2010, 07:49 PM
Paymemoney
quick complex number questions
Hi
Need help on the following:
1)if z is any cube root of unity then 1+z+z^2 must be.

A - 0
B - 1
C - 3
D - 0 or 3
E - 1 or 3

2) For any complex numbers z with conjugate $\displaystyle \overline{z}$, which of the following is not necessarily true?

A - $\displaystyle |z|=|\overline{z}|$

B - $\displaystyle \frac{z}{\overline{z}}$ is real

C - $\displaystyle \frac{z}{\overline{z}}$ is real

D - $\displaystyle z+\overline{z}$ is real

E - $\displaystyle |z/\overline{z}|$ is real

P.S
• Nov 7th 2010, 08:08 PM
pickslides
You can have a go yourself

say $\displaystyle z = a+bi$

Quote:

Originally Posted by Paymemoney

A - $\displaystyle |z|=|\overline{z}|$

Find

$\displaystyle |z| = |a+bi| = \sqrt{a^2+(b)^2}$ and $\displaystyle |\overline{z}|= |a-bi| = \sqrt{a^2+(-b)^2}$

are they the same?

Quote:

Originally Posted by Paymemoney

B - $\displaystyle \frac{z}{\overline{z}}$ is real

Is $\displaystyle \frac{a+bi}{a-bi}$ real?

Quote:

Originally Posted by Paymemoney

C - $\displaystyle \frac{z}{\overline{z}}$ is real

not sure how this one is different to the previous option?!

Quote:

Originally Posted by Paymemoney

D - $\displaystyle z+\overline{z}$ is real

evaluate $\displaystyle a+bi + (a-bi)$ is it real?

Quote:

Originally Posted by Paymemoney

E - $\displaystyle |z/\overline{z}|$ is real

Is $\displaystyle \left|\frac{a+bi}{a-bi}\right|$ real?
• Nov 7th 2010, 11:21 PM
mr fantastic
Quote:

Originally Posted by Paymemoney
Hi
Need help on the following:
1)if z is any cube root of unity then 1+z+z^2 must be.

A - 0
B - 1
C - 3
D - 0 or 3
E - 1 or 3

[snip]

Note that $\displaystyle z^3 = 1 \Rightarrow z^3 - 1 = 0 \Rightarrow (z - 1)(z^2 + z + 1) = 0 ....$