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# C3 Trigonometry Question Tweet

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1. C3 Trigonometry Question
Hey guys, stuck on q.8(ii)(b) on this paper:

So you need to use part 8(i), the answer to which is root 18 cos(x-0.785).

So for 8(ii)(b), here is my working:

8/(root 18 cos(x-0.785) ) = 8/9 root 6
therefore 8 = (root 18 cos (x-0.785).8 root 6) / 9
therefore 8 = (48 root 3 cos (x-0.785) ) / 9 = 8
therefore 48 root 3 cos(x-0.785) = 72
therefore cos(x-0.785) = (root 3) / 2
hence 3x - 0.785 = 1/6 pi
therefore x = 5/36 pi (this is what the expression simplifies to).

However, the answer is 1/36 pi, no idea where I've gone wrong...any ideas?

Pos rep for a clear explanation Cheers!
2. Re: C3 Trigonometry Question
instead of pi/6 use -pi/6 which has the same cos
3. Re: C3 Trigonometry Question
(Original post by TenOfThem)
instead of pi/6 use -pi/6 which has the same cos
ahh I see, how did u quickly realise that the cos of pi/6 is the same as the cos of -pi/6?
4. Re: C3 Trigonometry Question
(Original post by Next Level)
ahh I see, how did u quickly realise that the cos of pi/6 is the same as the cos of -pi/6?
CAST or graphs

whatever method you normally use to find the "other" solutions

For me it is CAST
5. Re: C3 Trigonometry Question
(Original post by TenOfThem)
CAST or graphs

whatever method you normally use to find the "other" solutions

For me it is CAST
ahh right usually for cos I'd just do 360 - (the other value), but thinking about a cast diagram the negative value of the first solution is the equivalent of that no?
6. Re: C3 Trigonometry Question
(Original post by Next Level)
ahh I see, how did u quickly realise that the cos of pi/6 is the same as the cos of -pi/6?
If you have studied C2, then you should be knowing that
7. Re: C3 Trigonometry Question
(Original post by Next Level)
ahh right usually for cos I'd just do 360 - (the other value), but thinking about a cast diagram the negative value of the first solution is the equivalent of that no?
Yes
8. Re: C3 Trigonometry Question
(Original post by Next Level)
ahh right usually for cos I'd just do 360 - (the other value), but thinking about a cast diagram the negative value of the first solution is the equivalent of that no?
For something like this it'd be better to be able to picture the cosine graph, there's perfect symmetry. As raheem94 has said cos(x) = cos(-x).

Remember for sine, sin(-x) = -sin(x)
9. Re: C3 Trigonometry Question
(Original post by raheem94)
If you have studied C2, then you should be knowing that
TBH you would have learnt this at GCSE
10. Re: C3 Trigonometry Question
(Original post by TenOfThem)
TBH you would have learnt this at GCSE
Is this taught at GCSE?

11. Re: C3 Trigonometry Question
Cheers guys!
12. Re: C3 Trigonometry Question
(Original post by raheem94)
Is this taught at GCSE?

yes

and the trig graphs ... using them to find other values

and sin^2 + cos^2 = 1

and sin/cos = tan

and the question you referred to earlier where you are given cos of an angle and expected to find sin or tan using the triangle
13. Re: C3 Trigonometry Question
(Original post by TenOfThem)
yes

and the trig graphs ... using them to find other values

and sin^2 + cos^2 = 1

and sin/cos = tan

and the question you referred to earlier where you are given cos of an angle and expected to find sin or tan using the triangle
This is on the Edexcel GCSE specification? I can't remember ever studying this at GCSE.
14. Re: C3 Trigonometry Question
(Original post by TenOfThem)
yes

and the trig graphs ... using them to find other values

and sin^2 + cos^2 = 1

and sin/cos = tan

and the question you referred to earlier where you are given cos of an angle and expected to find sin or tan using the triangle
GCSE is quite different to the GCE 'O' Level which i did.

My one didn't included anything from the list except the last point of using that triangle though i do believe that overall it was much tougher than GCSE. But as it was very old spec, so didn't had any good books to self-study it, and the grade boundaries were pathetic when i took the exam.
15. Re: C3 Trigonometry Question
(Original post by NutterFrutter)
This is on the Edexcel GCSE specification? I can't remember ever studying this at GCSE.
Trig graphs and the trig values of other angles so knowing cos(a) = cos(-a) is certainly there

When I referenced the others I was referring to questions that have been asked on AQA that expect students to use pythagoras and trig on the non-calculator paper