Posted by tabby on Friday, April 13, 2012 at 3:44pm.
Teach me how to express 5sqrt3  5i in polar form please.
I don't want you to do the work for me. Just show me the steps I need to do the work properly on my own. Otherwise I will not pass this class or the exam when I enter college, and I do not want to retake PreCalc.
Its a multiple choice homework question with answer choices:
a: 10(cos 11pi/6 + i sin 11pi/6)
b: 10(cos 11pi/6  i sin 11pi/6)
c: 5 (cos 11pi/6 + i sin 11pi/6)
d: 10(cos 5pi/3 + i sin 5pi/3)
I did as much work as I could guess at, and came up with r=10sqrt2,
arctan=1/sqrt3 which equals sqrt3/3.I know that sqrt3/3 = tan 30 and 210 degrees. But now I am stuck.
Where do I go from here  or is this even the right way to try to solve this equation? Please help me.

Trig/PreCalc  Reiny, Friday, April 13, 2012 at 8:49pm
start by graphing 5√3  5i
as (5√3, 5) in the argand plane
which would be in quadrant IV
r^2 = (5√3)^2 + (5)^2 = 100
r = 10
we also know that tanØ = 5/(5√3) = 1/√3
( I know that tan 30° = tan π/6 = 1/√3)
but my angle is in IV, so Ø = 11π/6 or 330°
so 5√3  5i = 10(cos 11π/6 + i sin 11π/6)
check:
RS = 10( √3/2 + i(1/2)
= 5√3  5i
= original complex number
so in summary
for a + bi
1. sketch (a,b) to see which quadrant you are in
2. evaluate r,
with r^2 = a^2 + b^2 > r =  √(a^2 + b^2)
3. from tan Ø = b/a find the acute angel Ø
 for I, Ø is that acute angle
 for II , Ø = π  acute angle
 for III , Ø = π + acute angle
 for IV, Ø = 2π  acute angle
a + bi = r( cosØ + isinØ)
use your calculator to verfiy your answer.
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