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# C3 Trigonometry Questions: help! Watch

1. zzz
2. (Original post by rahyahrah)
I have some (relatively) obscure questions about C3 Trigonometry:

When using the special triangles (eg: http://fouss.pbworks.com/f/special%20triangle.JPG), what do you do if the value you are using at the start is negative?
- For example, cos^(-1) of (-((root 3)/2)). By doing r3/2 seperately, I get pi/6. And my notes say that if it is negative you should take it away from pi to get the value. This gives 5pi/6 - which is correct. However, the same method doesn't work when using degrees (180 instead of pi), or when using radians with sin - I guessed this method only worked because of the symmetry of cosine? So how would I use trigonometry triangles to work out sin^(-1) of (-((root 3)/2))? I'm only really bothered about this method because it helps to work out ~exact~ values!

- I managed to simplify the equation to 1 = 2sin(x), which gives the answers as 0.52 and 2.62 - which are correct. But the answer book has two other values which I can't seem to get: 1.57 and 4.71! These correspond to sin(x)=1 and sin(x)=-1, how can these be attained from the original equation. Where have I gone wrong?

Thanks in advance and - yes - I am overzealous with the brackets! x
cot(x)-2 cos(x)=0

cos(x) (1/sin(x)-2)=0

cos x = 0 or sin(x)=1/2
3. zzz

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