Ptolemy a) Prove the sum formula, 120 crd(180º – (a + b)) = crd(180° – a) crd(180° – B) – crd(a) crd(B), using Ptolemy's formula on quadrilaterals inscribed in a circle. b) Use Ptolemy's difference formula to calculate crd(12°) and then apply the half-angle formula to calculate crd(6°) and crd(30). Note: Use decimal numerical system for your calculations. You may use that crd(72°) = 70; 32, 3. Compare your result to Ptolemy's: crd(3°) = 3; 8, 28 and crd(69) = 6; 16; 49.