**Sine Rule: **The sides of a triangle are proportional to the sines of the angles opposite to them.

a / sin A = b / sin B = c / sin C = 2R

(Or)

sin A /a = sin B /b = sin C/c = 1/2R

Where R is the circum radius of the circle.

**Ex: **If R = 3, a = 6 Then ∟A =?

a = 2Rsin A => 6 = 6 sin A

=> sin A = 1 => ∟A = 90⁰

**Cosine Rule: **The angle of a triangle are related to three sides of a triangle by cosine rule.

cos A = b^{2} + c^{2} – a^{2} / 2bc (or) a^{2} = b^{2} + c^{2} – 2bc cos A

cos B = a^{2} + c^{2} – b^{2} / 2ac (or) b^{2} = c^{2} + a^{2} – 2ac cos B

cos C = a^{2} + b^{2} – c^{2} / 2ab (or) c^{2} = a^{2} + b^{2} – 2ab cos C

**Ex: **If a^{2} + b^{2} = c^{2} find cos C?

**Sol: **cos C = a^{2} + b^{2} – c^{2} / 2ab = 0/ 2ab = 0 => ∟C = 90^{0}