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System Message

[00:00:00] *** Please note: All sessions are recorded for quality control. ***

Guest (Customer)

[00:00:00] How do you do this?
[ File > http://lhh.tutor.com/SharedSessionFiles/47182371-1b3c-467e-9bfe-16fa767400cb_untitled.JPG ]

Cameron S (Tutor)

[00:00:06] Welcome to Tutor.com !
Let's get started.

Guest (Customer)

[00:00:09] ok

[00:00:24] for number 7 i thought the
side length was 4-x^2

[00:00:39] and then i thought you would
integrate it from 0 to 2

[00:00:42] but thats wrong

Cameron S (Tutor)

[00:01:03] OK.

[00:01:11] Which method are you using, first?

[00:01:21] Disks or cylindrical shells?

Guest (Customer)

[00:01:38] i have no idea
i think its
disks

Cameron S (Tutor)

[00:01:53] OK, that's what I would go with...just wanted to make sure.

[00:01:55]

Guest (Customer)

[00:01:56] ok

Cameron S (Tutor)

[00:02:11] Let me put that on the board quickly.

Guest (Customer)

[00:02:14] ok

[00:03:15] do you want me to insert the
picture?

Cameron S (Tutor)

[00:03:44] There it is.

Guest (Customer)

[00:03:49] ok

Cameron S (Tutor)

[00:03:56] The picture in your file is very small for my old eyes

Guest (Customer)

[00:04:05] oh sorry

Cameron S (Tutor)

[00:04:57] First, it would go from 0 to 4 since we're integrating with respect to 'y'.

[00:05:11] Your method would work is we were revolving about the x-axis.

Guest (Customer)

[00:05:13] yes i have that part

Cameron S (Tutor)

[00:05:14] Make sense?

Guest (Customer)

[00:05:15]

Cameron S (Tutor)

[00:06:09] If that's a disk that I just drew, the radius would be the distance to the curve.

Guest (Customer)

[00:06:35] is the answer 8pi?

Cameron S (Tutor)

[00:06:46] Let's check.

[00:07:20] Nice job.

Guest (Customer)

[00:07:46] thanks! i realized my mistake i
was trying to do dx instead of
dy

Cameron S (Tutor)

[00:08:04] Yep, it's easy to make that mistake!

Guest (Customer)

[00:08:15] so how would you do the next
one?

Cameron S (Tutor)

[00:08:32] Would the next one be #8?

Guest (Customer)

[00:08:37] yes

Cameron S (Tutor)

[00:08:54] OK, let me paste in another graph.

Guest (Customer)

[00:08:57] ok

Cameron S (Tutor)

[00:10:47] That area about the x axis.

[00:10:53] Can you set up the integral?

[00:10:59] This one will be dx.

Guest (Customer)

[00:12:12] sorry i really dont know i
thought it would be pi times
6radical x

[00:12:24] but now im confused

Cameron S (Tutor)

[00:12:38] 6radical x is the radius...nice job.

[00:12:52] ...and, of course, that gets squared.

Guest (Customer)

[00:14:50] ok so i guess my confusion
comes from when do you know
if its dx or dy?

Cameron S (Tutor)

[00:15:01] Ahhh....good question.

[00:15:09] Using the disk method or the washer method...

[00:15:22] If you revolve about the x-axis, it's always dx.

[00:15:33] If you revolve about the y-axis, it's always dy.

Guest (Customer)

[00:15:38] ok

[00:16:30] so i got 18pi

Cameron S (Tutor)

[00:16:36] Right!

Guest (Customer)

[00:16:52] yay

Cameron S (Tutor)

[00:16:59] Nice job.

[00:17:06] Anything else?

Guest (Customer)

[00:17:12] number 10 pleaswe

[00:17:15] please

Cameron S (Tutor)

[00:17:23] OK, let me take a look.

[00:19:10] I'm trying to understand the wording here.

[00:19:26] The cross sections perpendicular to the x-axis...

[00:19:45] If those are semicircles...

[00:19:53] Not sure what they mean by that.

[00:20:26] They might mean the top half of a solid of revolution obtained by revolving that line about the x axis.

Guest (Customer)

[00:20:55] i thought they just meant use
the equation for the
semicircle in the integration

[00:21:09] like piside^2/2

Cameron S (Tutor)

[00:21:37] OK...that would be equivalent.

[00:21:39] Let's try it.

[00:22:33] The trick is finding the radius.

[00:22:39] Let me know if you need help with that part.

Guest (Customer)

[00:23:16] is the radius 6-2y?

Cameron S (Tutor)

[00:24:02] The radius will be y, but y has to be expressed in terms of x, since we're integrating using dx.

Guest (Customer)

[00:24:46] so its (6-x)/2

Cameron S (Tutor)

[00:24:53] Yes!

Guest (Customer)

[00:26:34] i dont think im getting the
right answer

Cameron S (Tutor)

[00:26:47] OK, I'll put the integral on the board.

Guest (Customer)

[00:26:49] ok

Cameron S (Tutor)

[00:27:31] So far so good?

Guest (Customer)

[00:27:48] yes

Cameron S (Tutor)

[00:27:55] OK, next step...

[00:28:51] Is that what you got?

Guest (Customer)

[00:29:03] no

[00:29:23] i got 18pi

Cameron S (Tutor)

[00:29:43] OK.

[00:29:51] Where did we diverge on this one?

Guest (Customer)

[00:30:11] ok well i just put this part in
my calculator.

Cameron S (Tutor)

[00:30:44] OK, let me try that too.

Guest (Customer)

[00:30:58] ok never mind i got 9pi

[00:31:01] sorry

Cameron S (Tutor)

[00:31:08] OK, no problem.

[00:31:23] Doesn't look like 9pi is a choice though.

Guest (Customer)

[00:33:14] ok i dont know how to fix it

Cameron S (Tutor)

[00:33:33] Me neither...I can't see any other way to interpret their question.

Guest (Customer)

[00:33:41] ok i think its a typo

[00:33:51] ill just ask my teacher

Cameron S (Tutor)

[00:33:56] Must be...our answer looks good.

[00:33:58] OK

Guest (Customer)

[00:34:08] thanks for your help! have a
great day!

Cameron S (Tutor)

[00:34:18] You're welcome. You too!

[00:34:23] Thanks for using Tutor.com.Â Please fill out the survey as you leave so we can learn how we can better help you in the future!Â