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# [CORE 3] Addition Formulae for Trig! watch

1. Hey guys,

Right, I've been studying Trig recently, in particular, the Addition Formula. I know how it works, it's just plugging two degrees into a formulae and BAM but I have a few questions if you brainy ones can answer, I'd really appreciate it.

Firstly, my friend said that you can only use 30 degrees, 45 degrees, 60 degrees and stuff.

To make 75 degrees you need to make A 45 degrees and B 30 degrees. Well, why can't you use A is 23 degrees and B is 52 degrees? or A is 11 degrees and B ios 64 degrees etc?

Secondly, I put all the junk into the calculator and it gives answers in a different format i.e.: Different roots and denominators. Would this matter in the exam? [EDEXCEL]
2. (Original post by Iceyy)
Hey guys,

Right, I've been studying Trig recently, in particular, the Addition Formula. I know how it works, it's just plugging two degrees into a formulae and BAM but I have a few questions if you brainy ones can answer, I'd really appreciate it.

Firstly, my friend said that you can only use 30 degrees, 45 degrees, 60 degrees and stuff.

To make 75 degrees you need to make A 45 degrees and B 30 degrees. Well, why can't you use A is 23 degrees and B is 52 degrees? or A is 11 degrees and B ios 64 degrees etc?

Secondly, I put all the junk into the calculator and it gives answers in a different format i.e.: Different roots and denominators. Would this matter in the exam? [EDEXCEL]
Firstly
Depends what the question is asking but if it says "show that ... ... " or "exact value" then it is looking for the trig functions of 30 and stuff to be used as they are the ones we know exact values of

Secondly
depends what form the answer is asking for
3. Yes, the compound angle formulas work for any two angles, not only 30, 45, and 60.

The problem is how they're used. For example, if the exercise was "Find the exact value of sin(75)", then yes you could expand it into sin(23+52)=sin(23)cos(52)+sin(52 )cos(23). But you don't know sine or cosine fo 23 or 52, so this expansion doesn't help much. However, if you expand it into sin(45+30)=sin(45)cos(30)+sin(30 )cos(45), then because you know the sine and cosine of 30 and 45 you can evaluate it into an expression of surds.

If you have a calculator, wouldn't you just put in sin(75)? They should still be equal, though, for example sin(75)=-0.3877816354 and sin(30)cos(45)+sin(45)cos(30)=-0.5190347378+0.1312531024=-0.3877816354

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