Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 2 - No. 41)
The formula to use when all three sides of a triangle are given to find an angle is:
a^2 = b^2 + c^2 - 2bc cos A
Area = 1/2 ab sinC
sinA/a = sinB/b
a^2 = b^2 + c^2
คำอธิบาย
The Cosine Rule, a^2 = b^2 + c^2 - 2bc cos A, is the correct formula to use