A b 2 2ab July 8, 2019 16 32 64 128 A b 2 2ab formula A b 2 2ab ab A 2 b 2 2abcosθ A 2 b 2 2ab cos A 2 b 2 2ab cos theta A^2 ×b^2 + 2ab cos A 2 b 2 2abcosθ proof A 2 b 2 2ab 1 A 2 b 2 2ab 25 A 2 b 2 2ab cos c A 2 b 2 2ab 0 A 2 b 2 2ab proof A 2 b 2 2ab 3
