Can any body help with this one.

Evaluate the following using De Moivres Theorm's.

(2+i)^3

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- Jul 29th 2010, 01:15 AMheatlyComplex Number--De Moivres Theorm
Can any body help with this one.

Evaluate the following using De Moivres Theorm's.

(2+i)^3 - Jul 29th 2010, 01:35 AMAckbeet
What have you done so far?

- Jul 29th 2010, 02:05 AMProve It
- Jul 29th 2010, 02:34 AMheatly
I did that and got,

|2+i| = sqrt 5

z=(sqrt(5))^3 (cos pi/2 + i sin pi/2)

......

Which gives...0+11.18i ???

The ans is z= 2 + 11i - Jul 29th 2010, 02:59 AMAckbeet
I don't think you computed the angle correctly. Try $\displaystyle \theta=\tan^{-1}(\text{Im}(z)/\text{Re}(z)).$

- Jul 29th 2010, 03:10 AMheatly
thankyou very much you are correct Ackbeet.

- Jul 29th 2010, 03:26 AMAckbeet
You're welcome. Have a good one!