Volumes of right cylinder cones??????????????????

Question

ok, i tried doing this question,

it is a right cylinder cone with height of 4 and radius of 3, im suppose to find the volume.

in my book it says the volume is v=1/3BH

so im not sure what the base is, but i thought it was the radius x 2

so i did v = 1/3 (6) (4), so i got 1/3 12 =4, but im doing multiple choice and all the answers are in pie form, these:

12pie

6 pie

18 pie

or 24 pie

i also have ANOTHERquestion

If volume= 100cubic cm and h = 12, find the radius. of a right circular cone

Answer 1

The base of a cone is the area of a circle, pi*r^2, where r is the radius. So the actual volume of the cone is:

V = 1/3(pi*r^2)h

V = 1/3(pi*3^2)4

V = 1/3(9pi)4

V = 3pi(4)

V = 12pi <===ANSWER

V = 1/3(pi*r^2)h

100 = 1/3pi*r^2(12)

100 = 4pi*r^2

25/pi = r^2

r = sqrt(25/pi) <===ANSWER

And it's spelled "pi," not "pie". ;)

Answer 2

B is the area of the base. pi(d) or 2(pi)r

Answer 3

base means the area of the base which is pi*r^2 so it would be 9pi*4/3=12pi

so for the next one it's just 100=12pi*r^2/3

100*3/12=pi* r^2

25=pi*r^2

and so on

Answer 4

V = 1/3 BH

B = pi r^2

B = pi (3)^2

B = 9pi

V = 1/3 (9pi) 4

V = 12 pi

for no.2

use the same equation

V = 1/3 BH

given V = 100 cubic cm

H = 12 cm

100 = 1/3 B(12)

100 = 4B

B = 25

B = pi r^2

25 = pi r^2

r^2 = 25/pi

r =sqrt(25)/sqrt(pi)

r =5/sqrt(pi)

r = 2.82 cm