Can't answer Complex number

  • #1

Homework Statement



Under the section of complex number, i faced 2 questions which i couldn't answer... Here they go...

-Show that x=1-2i is a root of the equation x3-3x2+7x-5=0. Hence, find all the roots of the equation.


-Express http://img832.imageshack.us/img832/7916/msp180019e92f47ie4d5afg.gif [Broken] in the form a+ib, where a>0.

Homework Equations



for both question, ''i'' represents imaginary number

The Attempt at a Solution



I tried both for hour but couldn't solve them...
 
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Answers and Replies

  • #2
Char. Limit
Gold Member
1,204
14
For the first one, you'll need polynomial long division. Do you know how to do that?

For the second one, I would first express your complex number in polar form.
 
  • #3
For the first one, you'll need polynomial long division. Do you know how to do that?

For the second one, I would first express your complex number in polar form.

Is it possible for you to explain briefly about polynomial long division and complex number in polar form... cause i have no ideas what are they... if can please show me step-by-step working... ><
 
  • #4
dextercioby
Science Advisor
Homework Helper
Insights Author
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Polynomial long division sounds nasty. I would use the fact that the 3rd order polynomial with real coefficients must necessarily have a real solution and the 2 complex ones are conjugate one to another.

As for the second point, i would have to find the A from

[tex] 1+i\sqrt{3} = A^2 = (a+ib)^2 [/tex]
 
  • #5
tiny-tim
Science Advisor
Homework Helper
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Hi Michael! :smile:
-Show that x=1-2i is a root of the equation x3-3x2+7x-5=0. Hence, find all the roots of the equation.

"Show" means that you can assume that it's the answer …

so just put it into the LHS, and see whether that equals 0 (ie, what is (1 - 2i)3 etc?) :wink:

(have a square-root: √ :wink:)

start by writing 1 + i√3 in the form re (that's polar form), ie r is the magnitude, and θ is the angle from the x-axis :smile:
 
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