When do we apply cosine rule in a non right angles triangle? What are the cases?
+23254 The Law of Cosines applies to every triangle.
There are two forms of the Law:
1) If you know two sides (sides b and c) and the included angle (∠C) and want to know the side opposite to the known angle (side c), use this form: c² = a² + b² - 2·a·b·cos·(∠C) or c = √[ a² + b² - 2·a·b·cos·(∠C) ].
2) If you know all three sides and want to know one of the angles, call the angle that you want to know ∠C and label the side opposite that angle c. The other two sides are a and b.
Now, use this form: cos(∠C) = (a² + b² - c²) / (2·a·b) or ∠C = cos-1[ (a² + b² - c²) / (2·a·b) ]
I'm pretty sure that cosine rule can be used for any triangle regardless of whether it is right-angled.
+23254 The Law of Cosines applies to every triangle.
There are two forms of the Law:
1) If you know two sides (sides b and c) and the included angle (∠C) and want to know the side opposite to the known angle (side c), use this form: c² = a² + b² - 2·a·b·cos·(∠C) or c = √[ a² + b² - 2·a·b·cos·(∠C) ].
2) If you know all three sides and want to know one of the angles, call the angle that you want to know ∠C and label the side opposite that angle c. The other two sides are a and b.
Now, use this form: cos(∠C) = (a² + b² - c²) / (2·a·b) or ∠C = cos-1[ (a² + b² - c²) / (2·a·b) ]
