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# M3 Jan 09 qu 6a watch

1. Here is the paper and the mark scheme:

http://www.edexcel.com/migrationdocu...e_20090129.pdf

http://www.edexcel.com/migrationdocu...s_20090312.pdf

Why does the mark scheme use an integral of y squared? According to the M3 textbook, for x, you use integral of xy,dx divided by integral of y, but the mark scheme uses y squared.

thanks for any help.
2. (Original post by IL9)
Here is the paper and the mark scheme:

http://www.edexcel.com/migrationdocu...e_20090129.pdf

http://www.edexcel.com/migrationdocu...s_20090312.pdf

Why does the mark scheme use an integral of y squared? According to the M3 textbook, for x, you use integral of xy,dx divided by integral of y, but the mark scheme uses y squared.

thanks for any help.
I think you might be confusing working out the moment of a flat surface with that of a solid.

The volume of an elementary vertical slice is "pi y^2 dx", so we have the integral of "pi x y^2 dx" for the moment.

They've left out the pi - presumably because it cancels laters with the denominator.
3. Ah correct, I was mixing up centre of mass of a lamina with that of a volume. Thanks for the help.

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