# Math Help - Complex Number--De Moivres Theorm

1. ## Complex Number--De Moivres Theorm

Can any body help with this one.

Evaluate the following using De Moivres Theorm's.
(2+i)^3

2. What have you done so far?

3. Originally Posted by heatly
Can any body help with this one.

Evaluate the following using De Moivres Theorm's.
(2+i)^3
Start by converting $2 + i$ to Polar form...

4. 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

5. I don't think you computed the angle correctly. Try $\theta=\tan^{-1}(\text{Im}(z)/\text{Re}(z)).$

6. thankyou very much you are correct Ackbeet.

7. You're welcome. Have a good one!