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# C3 trigonometry help watch

1. I do not understand c3 trig at all. My past teacher didn't teach me how to use cast diagrams yet my new teacher only teaches using cast diagrams. Is there a way of doing it using graphs and not cast? Please help. If there are any videos that don't use the cast diagrams at all I would be very grateful
3. (Original post by Loola1234)
I do not understand c3 trig at all. My past teacher didn't teach me how to use cast diagrams yet my new teacher only teaches using cast diagrams. Is there a way of doing it using graphs and not cast? Please help. If there are any videos that don't use the cast diagrams at all I would be very grateful
Perhaps you could give us an example of a problem you're trying to solve, and then someone can advise you of the best way to tackle it
4. My teacher always used cast diagrams to demonstrate things but I've always preferred using the graphs which is perfectly doable as long as you know all the correct values for 90, 180 etc. And even then you can work them out with a calculator as it's C3

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5. (Original post by davros)
Perhaps you could give us an example of a problem you're trying to solve, and then someone can advise you of the best way to tackle it

How would I do this question:
work out the exact value of sec210 without using cast or the triangle?
6. He uses cast
7. (Original post by Loola1234)
He uses cast
cast?
8. (Original post by smartalan73)
My teacher always used cast diagrams to demonstrate things but I've always preferred using the graphs which is perfectly doable as long as you know all the correct values for 90, 180 etc. And even then you can work them out with a calculator as it's C3

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How would I work out the exact value of sec 210 just using a graph?
9. (Original post by Loola1234)
I do not understand c3 trig at all. My past teacher didn't teach me how to use cast diagrams yet my new teacher only teaches using cast diagrams. Is there a way of doing it using graphs and not cast? Please help. If there are any videos that don't use the cast diagrams at all I would be very grateful
Hey, have you tried www.examsolutions.net? It's a great website for learning about A-level maths for each of the major exam boards. I would get you a direct link but my computer's being awfully slow...

I couldn't use the CAST diagram either but this is where I learned how to use them.
10. Damn, beat me to it. xD
11. (Original post by Loola1234)
He uses cast
Yes but if you look further on that website, he shows you how to use the CAST in a way most teachers don't. It's where I learned to use it.
12. (Original post by MathsAstronomy12)
cast?
Cast diagrams. He doesn't work out the questions using graphs
13. (Original post by Hydeman)
Hey, have you tried www.examsolutions.net? It's a great website for learning about A-level maths for each of the major exam boards. I would get you a direct link but my computer's being awfully slow...

I couldn't use the CAST diagram either but this is where I learned how to use them.
Thank you. So are you saying you can only do c3 trig using the cast diagram?
14. (Original post by Loola1234)
Thank you. So are you saying you can only do c3 trig using the cast diagram?
No, you can do it any way you like as long as you get the right answer. That's the best thing about maths A-level: as long as you show your working and get the right answer, the examiner doesn't give a damn whether you've used the method your teacher taught you or not.

It's helpful to know both ways just in case you end up finding that easier. That's what happened with me anyway. I didn't get it at all in my AS year, which unfortunately brought my C2 mark well below what it should have been but once I got it, it was great.
15. (Original post by Loola1234)
How would I do this question:
work out the exact value of sec210 without using cast or the triangle?
You know (or should do!) that sec x = 1/cos x.

So you want to be able to find cos 210 first.

There are various ways you can do this e.g. plot the cosine graph for 0 <= x <= 360 BUT you also need to know some of the useful periodic properties of cosine and some of the addition formulae.

For example, you know that cosine repeats every 360 degrees so that cos(360 + x) = cos x. That doesn't help you here because it will take you beyond 210 degrees, so the question is: can you reduce 210 to the sum of two angles that you know about?

You should know the basic sine and cosine of angles like 30, 45 and 60 as well as the special cases like 0, 90 and 180. So for your case it is natural to write 210 = 180 + 30 and then use the standard formula for cos(A + B) from your formula booklet.

Have a go at this and see if you can get the correct answer.
16. Surely the only time you need to use cast diagrams is when you've got something like cosx=0.25 and it wants all values of x for 0<x<360. In this case you put it in your calculator and get one answer (76) and then you would use a cast diagram to find the other answers. However you could do this with graph by drawing the graph for cosx between 0<x<360 , finding where 76 is and finding all other values that have the same y value (284).

I can't think of any other time in C3 when you would have to use cast diagrams.

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