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# Integrals watch

1. Im getting 4160, exactly double the answer in the book.

I found the area of the rectangle around it (12 x 9)
Then minused the definite integral from 6 to -6 of 1/4x^2
Then minused the definite integral of 2 to -2 of 1-(1/4)x^2 which is the bottom white part of the cross section

Then multiplied this by 60, the length.
2. (Original post by Feynboy)

Im getting 4160, exactly double the answer in the book.

I found the area of the rectangle around it (12 x 9)
Then minused the definite integral from 6 to -6 of 1/4x^2
Then minused the definite integral of 2 to -2 of 1-(1/4)x^2 which is the bottom white part of the cross section

Then multiplied this by 60, the length.
Me too :s

By the way, an easier way to do this is to write it in terms of x, or in other words invert the axes, so then it's just a basic curve with a single integral to get the answer.

I got:

Then the volume is

3. Also getting the same answer here.
If I recognise the book correctly as one of the old AQA Textbooks then there are usually a few incorrect answers in them, so yours is probably correct
4. (Original post by Pessimisterious)
Me too :s

By the way, an easier way to do this is to write it in terms of x, or in other words invert the axes, so then it's just a basic curve with a single integral to get the answer.

I got:

Then the volume is

Is the *2 because the integral only calculates the area to one side of the axis?
5. (Original post by Feynboy)
Is the *2 because the integral only calculates the area to one side of the axis?
Precisely

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