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# C3 trigonometry Watch

1. Hi guys,

Can someone please explain this to me?

I have never seen this before until the question that I just tried to do from the Edexcel textbook.

I would have thought sinx=cos(x-(pi/2))?

2. (Original post by jasminetwine)
Hi guys,

Can someone please explain this to me?

I have never seen this before until the question that I just tried to do from the Edexcel textbook.

I would have thought sinx=cos(x-(pi/2))?

Remember that so both are true.
3. (Original post by SeanFM)
Remember that so both are true.
👌 thank you

However, doesn't one shift the graph to the left and one to the right?
4. (Original post by jasminetwine)
Hi SeanFM

Thanks but unfortunately I can't read the symbols that you sent 🙈

Are you able to show it to me some other way?

Thanks so much
He's saying cos(x) = cos(-x).
5. (Original post by Zacken)
He's saying cos(x) = cos(-x).
thank you

However, doesn't one shift the graph to the left and one to the right?
6. (Original post by jasminetwine)
thank you

However, doesn't one shift the graph to the left and one to the right?
If you let x = pi/2 - theta

then -x = -(pi/2 - theta) = theta - pi/2.

So cos(pi/2 - theta) = cos(theta - pi/2).
7. (Original post by Zacken)
If you let x = pi/2 - theta

then -x = -(pi/2 - theta) = theta - pi/2.

So cos(pi/2 - theta) = cos(theta - pi/2).
Thanks Zacken,

I have followed the question through after what you said...

Would you be able to check my workings please?

Attachment 547181547183

I got 2 different answers, but the answers only have one 🙈

Thank you very much!
Attached Images

8. (Original post by jasminetwine)
Thanks Zacken,

I have followed the question through after what you said...

Would you be able to check my workings please?

Attachment 547181547183

I got 2 different answers, but the answers only have one 🙈

Thank you very much!
You need to throw away one of the answers because it is given that -pi/2 < y < pi/2

But pi/2 + arccos x > pi/2, so that doesn't work.

So the other one is correct.

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