Observation: 2 = 1 * 2, 2 + 4 = 6 = 2 * 3 = 12 = 3 * 4, 2 + 4 + 6 + 8 = 20 = 4 * 5,..., calculation: 2002 + 2004 + 2006 +... + 2050

Observation: 2 = 1 * 2, 2 + 4 = 6 = 2 * 3 = 12 = 3 * 4, 2 + 4 + 6 + 8 = 20 = 4 * 5,..., calculation: 2002 + 2004 + 2006 +... + 2050

=(2002+2050)*26/2=50650
Or = 2000 * n + n * (n + 1) = 2000 * 25 + 25 * 26 = 50650