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# C4 Integration Help Watch

1. a) use integration by parts to find the integral of xcos2x

b) hence, find the integral of xcos^2x

done part a, got xsinxcosx +0.5cos^2x + c but stuck on b

2. (Original post by Wiggledinho)
a) use integration by parts to find the integral of xcos2x

b) hence, find the integral of xcos^2x

done part a, got xsinxcosx +0.5cos^2x + c but stuck on b

Use a double angle formula for cos2x.
3. (Original post by morgan8002)
Use a double angle formula for cos2x.
wow thanks didnt see that haha cheers
4. (Original post by Wiggledinho)
wow thanks didnt see that haha cheers
Do you know which cos2x double angle formula to use?
5. (Original post by morgan8002)
Do you know which cos2x double angle formula to use?
trying it with cos2x=cos^2x-1...
6. (Original post by Wiggledinho)
trying it with cos2x=cos^2x-1...
Good that's the one you should use.
It's though.
7. (Original post by morgan8002)
Good that's the one you should use.
It's though.
i meant that! not sure how to account for the x at the start though?
8. (Original post by Wiggledinho)
i meant that! not sure how to account for the x at the start though?
You need to use your answer from the last part. How can you make this integral look like the one in the last part?
The double angle formula you suggested is the correct way to do it.
Don't worry about the x at this stage
9. (Original post by Wiggledinho)
i meant that! not sure how to account for the x at the start though?
Have you gotten any further with it?

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