Use the triangle angle sum: add the two known angles and subtract from 180°
Formula: third angle = 180° – angle 1 – angle 2
If all three sides are known, use the Law of Cosines
Formula: cos(A) = (b² + c² – a²) / (2bc)
Solve for the angle with inverse cosine: A = cos⁻¹[(b² + c² – a²) / (2bc)]
If one side and two angles are known, use the angle sum to find the missing angle
If a right triangle is involved, use trigonometric ratios: sine, cosine, or tangent
Formula: sin(θ) = opposite / hypotenuse
Formula: cos(θ) = adjacent / hypotenuse
Formula: tan(θ) = opposite / adjacent
Use inverse trig functions to find the angle
Formula: θ = sin⁻¹(opposite / hypotenuse)
Formula: θ = cos⁻¹(adjacent / hypotenuse)
Formula: θ = tan⁻¹(opposite / adjacent)