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# stuck on maths question watch

1. hey
I am wondering if anyone could help me on this maths question

solve this equation 4 sec 2x = -7

between 0°≤x≤360°

thanks
2. Turn sec into cos and try rearranging the equation into a form cos(2x) = something.
3. i got to cos 2x = -1/7 - don't know if this is right

what shall i do next?
4. (Original post by Lehmber Hussainpuri)
i got to cos 2x = -1/7 - don't know if this is right

what shall i do next?
4sec2x = -7
4/cos2x = -7
4 = -7cos2x
cos2x = -4/7

Do you know how to solve that? If not, think about how you'd solve this:

cos y = -4/7

If you know cos y = -4/7, how can you work out cos 2x = -4/7? Can you see a similarity about cos y and cos 2x in this case? They're equal. So y = 2x.
5. thanks i get it now
6. Also don't forget that the range of 2x is:

0≤2x≤720

So when solving:

cos 2x = -4/7

You need the solutions from 0 to 720 so that when you halve them (to get values for x) they will be between 0 and 360

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