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C4 Parametrics Area Under Curve

How did I get +24cos(4theta) instead of -24cos(4theta)? ImageUploadedByStudent Room1464009192.884314.jpg
ImageUploadedByStudent Room1464009214.317830.jpg


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Reply 1
Could you post the question?
Reply 2
Original post by TSRforum
How did I get +24cos(4theta) instead of -24cos(4theta)? ImageUploadedByStudent Room1464009192.884314.jpg
ImageUploadedByStudent Room1464009214.317830.jpg


Posted from TSR Mobile


Several things are wrong.

1. It should be 0π/6+48dθ\int_0^{\pi/6} + 48\cdots \, \mathrm{d} \theta

2. sinθsin3θ=12(cos2xcos4x)\sin \theta \sin 3\theta = \frac{1}{2}(\cos 2x - \cos 4x). i.e: cos(2x)=cos2xcos2x\cos(-2x) = \cos 2x \neq -\cos 2x as you seem to think.
Reply 3
Original post by Zacken
Several things are wrong.

1. It should be 0π/6+48dθ\int_0^{\pi/6} + 48\cdots \, \mathrm{d} \theta

2. sinθsin3θ=12(cos2xcos4x)\sin \theta \sin 3\theta = \frac{1}{2}(\cos 2x - \cos 4x). i.e: cos(2x)=cos2xcos2x\cos(-2x) = \cos 2x \neq -\cos 2x as you seem to think.


So when it says (A-B) in the trig identity I should look at that as the difference between A and B?


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Reply 4
Original post by TSRforum
So when it says (A-B) in the trig identity I should look at that as the difference between A and B?


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That's generally what subtraction means, yes.
Original post by Zacken
That's generally what subtraction means, yes.


Not sure, difference of xx and yy might mean xy|x-y|.

If by their last post OP means ABA-B should be taken as AB|A-B| then yes but only because cos(x)cos(x)\cos(x) \equiv \cos(-x).
(edited 7 years ago)
Reply 6
Original post by IrrationalRoot
Not sure, difference of xx and yy might mean xy|x-y|.

If by their last post OP means ABA-B should be taken as AB|A-B| then yes but only because cos(x)cos(x)\cos(x) \equiv \cos(-x).


Good point, I wasn't sure what the OP was asking.

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