# How to solve...?

cos(180+A) = -cosA

I did this…

cos180 x cosA - sin180 x sinaA = -cosA

But what do I do next? D:

Cos 180=-1
Sin 180=0

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.

I’m in 11th grade.

And thank you for your help.

What course?

He wasn’t thanking you Setz.

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.

I… I never said s/he was?

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:

I took a Functions and Relations (MCR3U1) course in grade 11; and it was considered pre-calc; is that it?.

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.

Oooh! Thank you, that’s very helpful!