# Algebra Question from Chapter 5 page 133 equation 5.56 | FX Book Questions | Forum

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Algebra Question from Chapter 5 page 133 equation 5.56
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January 3, 2015 - 1:34 am
Member Since: December 25, 2014
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Hey Will,

I have been going through your book and I am having a great time reading through it. I did not know where to place this topic so if it needs to be moved please do so.

I want to know how you derived equation 5.56 from equation 5.55. I understand that you multiplied complex conjugates to have the imaginary components disappear, but I cannot recall how to do so. I want to see the steps you took to get to 5.56 if possible.

Thank you

January 4, 2015 - 8:38 pm
Member Since: January 29, 2017
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The steps are more logical than symbolic-math. First, obviously you let a0 = 1.0 since it is just a scaling factor. Then, you wind up with an algebra equation not unlike x^2 + x + 1, which you factor into its parts. In algebra class, you are given pure real equations like

ax^2 + bx + 1 = 0

and then you factor to find the roots. The generalized solution is

(x - C)(x - D) = 0 and it is up to you to figure out C and D. You probably did this by trial and error back in algebra class (high-school as we call it in the states).

But, for a complex equation H(z), you still decompose the equation as

(x - C)(x - D)

Where C and D are complex values with real and imaginary parts. Since after multiplication, the final equation has only real coefficients (marked as a and b in the simple equation above) then C and D must be complex conjugates of each other, or D = C*

C = a + jb
C* = a - jb

When you multiply, the imaginary component disappears. To do this by hand, use the old FOIL rule (first, inside, outside, last) on (x - C)(x - C*)

x^2 - Cx - C*x + (C)(C*) and substitute a+jb and a-jb above.

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January 6, 2015 - 1:34 am
Member Since: December 25, 2014
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Thank you so much for the clarification. I did not get the part how the a0 coefficient was eliminated. And your explanation for the second part that was a great deal of help as well.

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