Maths nerds!

How can you tell when too use cosine rule and sine rule??

Use the SOH CAH TOA rule:
use Sine rule when you have the angle, the opposite side or the hypotenuse (longest side)
use Cosine rule when you have the angle, the adjacent side or the hypotenuse (ie both sides touching the angle)

I'm guessing you mean for non-right angled triangles not what post #2 is saying?

Cosine rule: use to find an angle when you have three sides [always find the one opposite the longest side as it's the only one that could be obtuse]
Cosine rule: Use to find the third side when you know two sides and the angle between them

Sine rule: You need a side and the angle opposite that side - you can then find a second angle [the one opposite the other side you know]
Sine rule: Find a second side when you are given two angles [you can then find the third by using the fact the angles in a traingle add to 180] again use a side and angle opposite

Yes Thank You!!

We usually have a lesson on this and drawa few diagrams to go with the explanation above. You should b=never need to use the cosine rule more than once.

Do be aware of the ambiguous case linked to the sine rule.

I learnt it as, if its not a right angled triangle, rule out sohcahtoa, then see if sine rule works for it, if all else fails, cosine rule is your *last resort*.

That's a slower way to do it - it's obvious if you don't know any angles that it's not sine rule