I have a triangle with 

Side A = 2
Side B = 6

Angle c (angle opposite Side C) = 105°

I want to find side C

My first though was to use the SOH CAH TOA rule

In this case we have the adjacent and hypotenuse of angle c so I use CAH (cosθ = A/H)

Because 105° is in the 2nd quadrant it will be -cos

So -cos 105° = 2/C

Therefore C = 2/-cos 105°

```
        = 7.73

```

I thought this was the correct way to do it because that's all we have been learning in maths.

But in physics we are taught to use C = √(A²+B² - 2ABcosθ) which gives a different answer.

So which do I use/which is more accurate?