In the triangle ABC, a, B and C are the opposite sides of ∠ a, B and C respectively. Try to compare the square of a-b-c and the square of 2BC

In the triangle ABC, a, B and C are the opposite sides of ∠ a, B and C respectively. Try to compare the square of a-b-c and the square of 2BC

From the cosine theorem, - 1=