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# Edexcel C2 Trigonometry Help

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1. I'm doing a Silver Tiered C2 Paper
https://a44694f152ee562e38a9454969d2...%20Edexcel.pdf
Need help on Question 4. I got 3 out of 7 marks on this question and this is what I did to achieve that:
cos (3x-10) = -0.4
3x-10 = cos^-1(-0.4) = 113.6
3x-10 = 113.6
3x = 113.6+10
3x = 123.6
x = 41.2

I don't understand the steps after that would help me achieve the additional 4 marks. I know what to do after, just don't understand why we need to do that, if that makes sense...?
2. There's more than one answer in the specified range. You only put one answer.
3. (Original post by ravioliyears)
...
You're asked to find all the solutions. You've only found one.

It's kind of like me asking you to solve and you saying, . I wouldn't give you the marks because you haven't given me all the solutions ().

That's the case here.

If then is a solution, but you need to find the others.

Here's an image to convince you:

The solutions are the points of intersection and you've only found one of them.

Remember that if is a solution, then so is and , etc...

Because is a 2pi periodic function, that is, if you add 2pi to its argument, it gives the same thing.
4. (Original post by B_9710)
There's more than one answer in the specified range. You only put one answer.
I know that. What I mean is I don't get why we have to do:
3x-10 = 360-113.58
and
3x-10 = 360+113.58
5. (Original post by ravioliyears)
I know that. What I mean is I don't get why we have to do:
3x-10 = 360-113.58
and
3x-10 = 360+113.58
Well . Draw the graph of y=cosx to try and see why.
6. (Original post by ravioliyears)
I know that. What I mean is I don't get why we have to do:
3x-10 = 360-113.58
and
3x-10 = 360+113.58
You might want to watch this and then this.
7. (Original post by ravioliyears)
I know that. What I mean is I don't get why we have to do:
3x-10 = 360-113.58
and
3x-10 = 360+113.58
That's how you find the other answers for what can be.

You ± 360
8. (Original post by Zacken)
You might want to watch this and then this.
Thanks. Not a fan of using the CAST method. How could I answer it graphically? Sorry - I feel like I'm asking such obvious questions that I should know the answer to.
9. (Original post by ravioliyears)
Thanks. Not a fan of using the CAST method. How could I answer it graphically? Sorry - I feel like I'm asking such obvious questions that I should know the answer to.
No worries. I avoid the CAST method as well. You should be able to see from the graph that (and for that matter) are periodic functions. That is, it repeats itself after every radians. So if you have then not only is a solution but so is and indeed, , ... you get the gist.

Also, with - the graph should reveal that so if is a solution,then so is .
10. (Original post by Zacken)
No worries. I avoid the CAST method as well. You should be able to see from the graph that (and for that matter) are periodic functions. That is, it repeats itself after every radians. So if you have then not only is a solution but so is and indeed, , ... you get the gist.

Also, with - the graph should reveal that so if is a solution,then so is .
Thanks, Zain! I get it now Thanks again
11. (Original post by ravioliyears)
Thanks, Zain! I get it now Thanks again
No worries. :-)

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