Astronomy: at is closest approach, mars is about 5.6x10^7 km from earth and its apparent size is about 0.00012 radians. what is the approximate diameter of mars?

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.