You are Here: Home >< Maths

# Volume through rotation of 360 watch

1. y= -5/7x + 60/7

lower limit = 5

upper limit = 12

I use the pi times (integral of y^2 WRS x and substituting upper and lower limits.

When I try this question, I get the integral of y^2 as 25/147x^3 - 600/98x^2 + 360/49x

I can tell this is wrong, and when I substitute in the limits, I get a negative number.

I do not want to use the volume of cone formula

What mistake am I making if you know and could you post your working out? I already know the answer but not how to get it. It's 175pi/3
2. One of your terms is incorrect.

We have

3. (Original post by Regret786)
y= -5/7x + 60/7

lower limit = 5

upper limit = 12

I use the pi times (integral of y^2 WRS x and substituting upper and lower limits.

When I try this question, I get the integral of y^2 as 25/147x^3 - 600/98x^2 + 360/49x

I can tell this is wrong, and when I substitute in the limits, I get a negative number.

I do not want to use the volume of cone formula

What mistake am I making if you know and could you post your working out? I already know the answer but not how to get it. It's 175pi/3
Check
4. (Original post by Regret786)
y= -5/7x + 60/7

lower limit = 5

upper limit = 12

I use the pi times (integral of y^2 WRS x and substituting upper and lower limits.

When I try this question, I get the integral of y^2 as 25/147x^3 - 600/98x^2 + 360/49x

I can tell this is wrong, and when I substitute in the limits, I get a negative number.

I do not want to use the volume of cone formula

What mistake am I making if you know and could you post your working out? I already know the answer but not how to get it. It's 175pi/3
I suspect your y^2 is wrong because the x term in your integral is.
You can make your life easier and less prone to errors if you factorise out the 5/7
This leads to 5/7(-x+12) and then V = pi x integral of [5/7(-x+12)]^2 = 25pi/49 x integral of (-x+12)^2 dx

Posted from TSR Mobile
5. Yeah I got it now guys, thanks a lot.

I made a mistake squaring 60/7. I forgetfully wrote 360/49 instead of 3600/49.

I had been looking for my mistake for quite a while now, can't believe it was something so small.

Fixing the mistake gives the correct answer.

Thanks again
6. I am just curious, how can the problem even be solved without the axis of revolution being stated?

Peace.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 19, 2016
Today on TSR

### University open days

1. University of Cambridge
Wed, 26 Sep '18
2. Norwich University of the Arts
Fri, 28 Sep '18
3. Edge Hill University
Faculty of Health and Social Care Undergraduate
Sat, 29 Sep '18
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams
{"found_math": false, "script_math_tex": 0, "script_math_asciimath": 0, "math_annotations": 0, "math_alttext": 0, "mathml": 0, "mathjax_tag": 0, "mathjax_inline_tex": 0, "mathjax_display_tex": 0, "mathjax_asciimath": 0, "img_math": 0, "codecogs_latex": 0, "wp_latex": 0, "mimetex.cgi": 0, "/images/math/codecogs": 0, "mathtex.cgi": 0, "katex": 0, "math-container": 0, "wp-katex-eq": 0, "align": 0, "equation": 0, "x-ck12": 0, "texerror": 0}