Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 1 - No. 16)
What do you need to know to use the cosine rule to find an angle in a triangle?
Two sides and the included angle
All three sides
Two angles and a side
Only the area of a triangle
Explication
To calculate an angle using the cosine rule, you need the lengths of all three sides.