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[efyyness]

Hi guys. I would really appreciate it if you could help me out. You see, we're doing trigonometric identities in my pre-calculus class, and for the most part I understand, but I'm stuck on the following equations:

1) Law of Cosines
You have a triangle with a=24 b=19 c=29 and you need to solve it (get all the angles) with the formula a2 = b2 + c2I

Comments

[deceptiverose]

Let me show you an example of sorts: You start with

a2 = b2 + c2 - 2bcCosA
Subtract b2 + c2 from both sides

a2 - b2 - c2 = -2bcCosA

Divide both sides by -2bc

(a2 - b2 - c2)/(-2bc) = CosA

Take Cos inverse of that:

Arccos[(a2 - b2 - c2)/(-2bc)] = A

Plug that into your calculator (with the appropriate numbers) and you get the answers you're looking for. The PreCalc book I teach out of actually gives that formula too.

For the second one, I'm not sure what it's looking for but I would look at the sum and difference identities. Good luck!

[efyyness]

Oh, I'm sorry. Completely spaced on what you're actually supposed to do with #2. :/ Sorry. It's proving the identity.

Thank you so much! You're a great help. Hopefully I ace my exam :D

[deceptiverose]

For #2 to prove it, I'll give you a hint:
You want to use the Sum and Difference Theorems and Cofunctions. That's all you need to prove that they are the same.

[efyyness]

Oh-hooo. Now I get it. Thank you for your help!!

[deceptiverose]

No problem! Glad I could help!

[efyyness]

So, I took the exam today and got the Law of Cosines question correct (thanks to you!) but I apparently couldn't remember the proof one. :D But I really appreciate this.