Bonjour,
a) 360/840
360 = 180 * 2
360 = 90 * 2 * 2
360 = 45 * 2 * 2 * 2
360 = 9 * 5 * 2 * 2 * 2
840 = 420 * 2
840 = 210 * 2 * 2
840 = 30 * 7 * 2 * 2
840 = 10 * 3 * 7 * 2 * 2
840 = 5 * 2 * 3 * 7 * 2 * 2
360/840 = (3 * 3 * 5 * 2 * 2 * 2)/(5 * 2 * 3 * 7 * 2 * 2) = 3/7