Question: Dutch math book, the volume of a function rotated around an axis.

Hello, ive got another question. Im in the last chapter of this book, and i hope i can finish the book this month. So i can start with part 2 of the series of applied math for higher job education. The first book is basically the freshmans year worth of math, and im completing it within 2 months. So im happy with that. :) 

Okey here is the question: 
Draw half a circle x^2+y^2=4, with y >= (bigger or equal to) 0. 

So how do i make half a circle instead of the whole circle? 

The rest of the text says:
Which body is created if the area bordered by the x-axis and this half circle is revolved around the x-axis?
Calculate the volume of this body. 


The Function

smartplot(x^2+y^2 = 4)




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