Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 6 - No. 13)

If you are given a triangle with sides a, b, and c and angle A, which formula is best used to calculate side a?

a² = b² + c² - 2bc cos A

a² = b² + c² + 2bc cos A

a² = b² - c² - 2bc cos A

a² = c² - b² + 2bc cos A

The correct formula is a² = b² + c² - 2bc cos A.