# How to solve...?
**URL:** https://discourse.rpgclassics.com/t/how-to-solve/16377
**Category:** RPGC Main Forum
**Created:** [February 7, 2006, 7:57pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377 "2006-02-07T19:57:03Z")
**Posts on this page:** 12
**Page:** 1
### Author:  [@defectivesonydvd](https://discourse.rpgclassics.com/u/defectivesonydvd)
#### Post date: [February 7, 2006, 7:57pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/1 "2006-02-07T19:57:03Z")
cos(180+A) = -cosA
I did this…
cos180 x cosA - sin180 x sinaA = -cosA
But what do I do next? D:
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### Author:  [@Gamer5389](https://discourse.rpgclassics.com/u/Gamer5389)
#### Post date: [February 7, 2006, 8:31pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/2 "2006-02-07T20:31:39Z")
Cos 180=-1
Sin 180=0
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### Author:  [@Setz](https://discourse.rpgclassics.com/u/Setz)
#### Post date: [February 7, 2006, 8:34pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/3 "2006-02-07T20:34:01Z")
Just wondering, but what grade are you in? I’m not implying you’re unintelligent by this question, I’m just wondering when a student would learn this stuff. I learned it in grade 9, but might be different for everyone.
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### Author:  [@defectivesonydvd](https://discourse.rpgclassics.com/u/defectivesonydvd)
#### Post date: [February 7, 2006, 10:08pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/4 "2006-02-07T22:08:24Z")
I’m in 11th grade.
And thank you for your help.
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### Author:  [@Setz](https://discourse.rpgclassics.com/u/Setz)
#### Post date: [February 8, 2006, 4:46am UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/5 "2006-02-08T04:46:14Z")
What course?
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### Author:  [@Sinistral](https://discourse.rpgclassics.com/u/Sinistral)
#### Post date: [February 8, 2006, 5:41am UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/6 "2006-02-08T05:41:06Z")
He wasn’t thanking you Setz.
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### Author:  [@Nebagram](https://discourse.rpgclassics.com/u/Nebagram)
#### Post date: [February 8, 2006, 7:47am UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/7 "2006-02-08T07:47:17Z")
easy.
cos(a+b) = (cos a)\*(cos b)
cos 180=-1 therefore cos(180+A)= (cos 180)_(cos A)= -1_(cos A)= -cosA
compound angle formulae man, gonna come in handy if you want to do pure maths at a higher level.
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### Author:  [@Setz](https://discourse.rpgclassics.com/u/Setz)
#### Post date: [February 8, 2006, 2:23pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/8 "2006-02-08T14:23:56Z")
> [@Sinistral](#):
>
> He wasn’t thanking you Setz.
I… I never said s/he was?
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### Author:  [@defectivesonydvd](https://discourse.rpgclassics.com/u/defectivesonydvd)
#### Post date: [February 8, 2006, 6:23pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/9 "2006-02-08T18:23:02Z")
It’s Pre-Calculus.
When you’re solving trigonometric equations, how do you know which quadrant it’s in?
For example:
2cosx+1=0
I know it’d be cosx=-1/2, but yeah. Maybe it’s easy but I’m stupid and don’t know. D:
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### Author:  [@Setz](https://discourse.rpgclassics.com/u/Setz)
#### Post date: [February 8, 2006, 6:49pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/10 "2006-02-08T18:49:42Z")
I took a Functions and Relations (MCR3U1) course in grade 11; and it was considered pre-calc; is that it?.
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### Author:  [@Sinistral](https://discourse.rpgclassics.com/u/Sinistral)
#### Post date: [February 8, 2006, 6:57pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/11 "2006-02-08T18:57:48Z")
To know which quadrant, ask yourself in which part of the quadrant X is negative. If in doubt, draw a circule, draw a big + in the middle to split it into quandrants and look where X is negative :P.
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### Author:  [@defectivesonydvd](https://discourse.rpgclassics.com/u/defectivesonydvd)
#### Post date: [February 8, 2006, 7:41pm UTC](https://discourse.rpgclassics.com/t/how-to-solve/16377/12 "2006-02-08T19:41:26Z")
Oooh! Thank you, that’s very helpful! 😃