Question

Can someone have a look at the attached problem . I get the answer 112pi/15 but the document states its 64pi/15, and I can't see where I've gone wrong.
Graph:http://imgur.com/epnj7
Problem:http://imgur.com/tgGbq

Answer 1

i dont think your gonna get any new answers from the last few time it was posted :)

Answer 2

pi {S} [outer radius]^2 - [inner radius]^2 dx; from 0 to 4

outer radius = 2;

inner radius = 2-(1/8)x^2

right?

Answer 3

http://www.wolframalpha.com/input/?i=int%28pi%282^2+-+%282-x^2%2F8%29^2%29%29dx+from+0+to+4

i see the dilemma :)

Answer 4

right

Answer 5

so i got the integral

pi (2^2 - (2 - 1/8 x^2)^2)

Answer 6

is that correct?

Answer 7

i had not used the washer method before - ok - thanx

Answer 8

that appears to be correct; washer is just the disc method subtracted from the disc method ...

Answer 9

yes

Answer 10

I see your problem, Jimmy. You're using the wrong value for r.

Answer 11

R=2, but r != 2-(1/8)x^2

Answer 12

Oh, actually, it totally should...

But if you use just (1/8)x^2 as r, it gives the answer they're looking for.

http://www.wolframalpha.com/input/?i=int%28pi%282^2+-+%28%28x^2%29%2F8%29^2%29%29dx+from+0+to+4

Answer 13

Could be their mistake.

Answer 14

Yeah, I think you got it right the first time.

Answer 15

yes -looks like its their mistake

Answer 16

i was'nt too confident as i hadn't used the washer method before but as amist said its no big deal,

Answer 17

Right.

Answer 18

i think i see it right :)