# Beginners Complex Analysis

• August 26th 2008, 02:45 PM
Beginners Complex Analysis
$z=1+2i, w=2-i, \zeta=4+3i$

(1) Use the quadratic formula to solve these equations; express the answers as complex numbers.

(a) $z^2+36=0$

(b) $2z^2+2z+5=0$

I have more of these but I will try them on my own after I receive help on these.

The number x is called the real part of z and is writtern x = Re z. The number y, despite the fact that it is also a real number, is called the imaginary part of z and is writter y = Im z.

(2) Find Re(1/z) and Im(1/z) if z= x + iy, z $\not=0$. Show that Re(iz)= -Im z and Im(iz) = Re z.

If further explanation is needed, please let me know. Thanks for the help!
• August 26th 2008, 03:34 PM
ThePerfectHacker
What did you do?
These look straightforward.
• August 26th 2008, 03:51 PM
Is 1(a)'s answer -4 plus or minus 24i over 72?

Simplified to -1 +- 24i over 18?

If this is right, then I know how to do these, lol.
• August 26th 2008, 04:00 PM
ThePerfectHacker
Quote:

Originally Posted by shadow_2145
Is 1(a)'s answer -4 plus or minus 24i over 72?

Simplified to -1 +- 24i over 18?

If this is right, then I know how to do these, lol.

This one is $z^2+36=0$ which becomes $z^2 = -36$.
The solutions are $z=\pm 6i$.
• August 26th 2008, 04:50 PM
I don't understand how to use the quadratic formula to find these answers. Thanks though.
• August 27th 2008, 02:26 AM
Moo
Quote:

Originally Posted by shadow_2145
I don't understand how to use the quadratic formula to find these answers. Thanks though.

You know that $i^2=-1$

$z^2+36$ looks like a lot $a^2-b^2=(a-b)(a+b)$.

So write : $z^2+36=z^2-(-36)=z^2-(6i)^2$. Here is your quadratic formula (Tongueout)

Quote:

(b) $2z^2+2z+5=0$
Just use the discriminant method.

In $ax^2+bx+c=0$, $x=\frac{-b\pm \sqrt{b^2-4ac}}{2a}$

Here, I'm pretty sure $b^2-4ac<0$ and this is where imaginary part intervenes.

Quote:

The number x is called the real part of z and is writtern x = Re z. The number y, despite the fact that it is also a real number, is called the imaginary part of z and is writter y = Im z.

(2) Find Re(1/z) and Im(1/z) if z= x + iy, z\not=0.
$\frac 1z=\frac 1{x+iy}$

When you have a complex number in a denominator, you often have to multiply it by its conjugate (called $\overline{z}$), that is $x-iy$ and by using the quadratic formula $(a-b)(a+b)=a^2-b^2$, we get $(x+iy)(x-iy)=x^2-(iy)^2=x^2+y^2$, which is a real number.

So $\frac 1{x+iy}=\frac 1{x+iy} \cdot \frac{x-iy}{x-iy}$

Can you go from here ?

Quote:

Show that Re(iz)= -Im z and Im(iz) = Re z.
What is i*z ?
What do you conclude ?

You gotta be able to do these (in a first time, to "know"), because it's quite a basis of the manipulation of complex numbers...
• August 27th 2008, 07:52 AM
mr fantastic
Quote:

Originally Posted by shadow_2145
I don't understand how to use the quadratic formula to find these answers. Thanks though.

If you showed all your working I'm sure your problem applying the quadratic formula could be easily resolved.
• August 28th 2008, 10:45 AM