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I'm hating PC3 right now! I'm trying to differentiate but I'm having a bit of trouble. When you differentiate cos2x you end up with -2sin2x don't you? I have to differentiate y=3sin2x+cos2x, which I have as 6cos2x-2sin2x. Then I have to prove that at the stationary points tan2x=3. How do I do the last bit?

Thanks

Thanks

Stationary points occur when dy/dx = 0 i.e. 6cos 2x - 2sin 2x = 0

Now, so long as cos 2x doesn't equal 0 you can divide by it to get 6 = 2tan 2x or tan 2x = 3 as required.

To finish off, just confirm that IF cos 2x does equal 0 then you don't get a stationary point, i.e. you haven't inadvertently eliminated any stationary points by dividing by cos 2x.

Now, so long as cos 2x doesn't equal 0 you can divide by it to get 6 = 2tan 2x or tan 2x = 3 as required.

To finish off, just confirm that IF cos 2x does equal 0 then you don't get a stationary point, i.e. you haven't inadvertently eliminated any stationary points by dividing by cos 2x.

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