hi!

can i know how to prove that the cosine rule is true for ∏/2≤ x ≤∏ for

c^2 = a^2 + b^2 - 2ab cos x?

i want to show that it is true for 0 ≤ x ≤∏ and have proven that it is true for 0≤x≤∏/2 but i dont know how to prove the other part..

thanks!!!