Trigonometric Functions - finding the surface Area(found already) and volume of cone

This question is way over my head, its the last question in my txt book for this chapter and the only one I dont fully understand and get.

Q. 10. A sector of a circle with radius 5cm and an angle of PI/3(as in u know the mathematical pi) subtended at the centre is cut out of cardboard. It is then curved around to form a cone. Find its exact surface area and volume.

answer for surface area is

25 PI / 6

(ive figured surface area out already)

my working

http://quickimg.com/uploads/4c8b389b...8565490ed1.jpg

ok i know the exact value of the surface area, but how do i get the volume?

the answer in the back for volume is

V = 125 http://www.epa.gov/waterscience/itm/ITM/sqroot.gif 35 PI

----------------

648 cm cubed

the decimal form of this answer is = 3.585245522

(not 35 is only under the square root just to clarify)

so just figuring out the volume of this stupid cut out sector is my problem now : (

thx all help appreciated