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# C3 Trigonometry Question watch

1. Hi. I get how to do this question, it's just that I'm stuck on a little part of it, which ruins my answer. Here goes:

Given that where is reflex, and where obtuse, calculate the value of .

From this, I can conclude that and that .

*** Am I right here to say it is negative as is in the second quadrant and so cosine is negative.

***This is where I'm stuck - Since it is a reflex angle, it covers both quadrants 3 and 4. So how do I know if it positive or negative as the third quadrant is negative for cosine but the fourth is positive. Hope I'm making sense here!?
2. The fact that A is a reflex angle and its tan is positive is sufficient to narrow it down to one quadrant. Can you work it out from that?
3. (Original post by ghostwalker)
The fact that A is a reflex angle and its tan is positive is sufficient to narrow it down to one quadrant. Can you work it out from that?
Oh thanks!
So we're talking about the third quadrant here, which means cosine is negative.
Thanks again.

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