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C3 trigonometry

Hi guys,

Can someone please explain this to me?

image.png
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))?

Thanks in advance!
Original post by jasminetwine
Hi guys,

Can someone please explain this to me?

image.png
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))?

Thanks in advance!


Remember that cos \theta = cos{-\theta} so both are true.
(edited 7 years ago)
Original post by SeanFM
Remember that cos \theta = cos{-\theta}
so both are true.

👌 thank you

However, doesn't one shift the graph to the left and one to the right?
(edited 7 years ago)
Reply 3
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).
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?
Reply 5
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).
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...

image.jpeg

Would you be able to check my workings please?

Attachment not found


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

Thank you very much!
Reply 7
Original post by jasminetwine
Thanks Zacken,

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

image.jpeg

Would you be able to check my workings please?

Attachment not found


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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