1.) D = 2pi * d * A / 360 degrees; Where: pi = 3.14; d = distance or 56,000,000 km (5.6x10^7 to standard notation) A = angular size in degrees or .00688 degrees (.00012 radian to degrees)
D = 2(3.14) * 56,000,000 * .00688 / 360 = 6.28 * 385,280 / 360 = 2,419,558.4 / 360 D = 6,720.996 or 6,721
Therefore, 6,721 km. is the approximate diameter of Mars.