# Homework Help: Integration to volume and Demoivre's Theorem

1. Mar 25, 2013

### jackscholar

1. The problem statement, all variables and given/known data
1)I need to find the volume of the shape attached.
2) Solve cos5θ in terms of cos only.

3. The attempt at a solution
1) I believe the shape is a cylinder that has a radius which decreases with height. Does this mean i integrate the volume of a cylinder in the equation:
V=∏∫y^2 dh where y represents the first circles radius of 10cm, and the limits of integration are 5 and 0 for the upper and lower bounds respectively?
2) My teacher has told me to use demoivre's theorem such that, cos5θ=(cosθ+isinθ)^5, and this is expanded using binomial distribution. From here any terms that contain an imaginary number are eliminated from the equation? I'm not sure if they are right and i'm even more confused as to where to go from there.

Any help is highly appreciated!

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Last edited: Mar 25, 2013