Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 6 - No. 35)
What do you need to know to use the cosine rule for finding an angle in a triangle?
All three sides of the triangle.
Two sides of the triangle and the included angle.
Two sides and one of the non-included angles.
The area of the triangle.
Paliwanag
You need to know all three sides to find the angle, when using the cosine rule.