# Evaluating complex multiplication?

#### Loren Booda

How might one comprehend the product of complex numbers

N
[pi](an+ibn)=C
n=1

such as by representation with vectors in the complex plane, or algebraic simplification? Specifically, I would like to know the value Re(C).

#### sridhar_n

Hi

Any complex number a+ib can be represented as R(& theta). Where R is the magnitude and & theta is the phase. So convert the complex number into R(& theta) form and multiply. The R parts multiply while the angle parts add up.

e.g. R(& theta)* P(& Theta) = R*P(& theta + & Theta)

Got it???

Sridhar

#### Loren Booda

A bit rough, sridhar, but helpful in jogging my memory.

Can you or another be more mathematical in regard to the transform involved?

Is it tan-1(b/a)=[the] and r=(a2+b2)1/2?

#### Hurkyl

Staff Emeritus
Gold Member
Right...ish

if (a + bi) = r exp(i&theta;), then it is true that

r = (a^2 + b^2)^(1/2)
and
tan &theta; = b/a

But you have to make sure that &theta; is in the correct quadrant. (iow you might have to add &pi;).

#### Loren Booda

Hurkyl,

Is there a simplifying (exact) identity for the arithmetic series

N
[sum]tan-1(bn/an)
n=1

and for the geometric series

N
[pi](an2+bn2)1/2
n=1

or, more importantly, for my original statement concerning Re(C)?

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