f)
25 + 50 + 75 + 100 +....+ 400 =
= 25(1 + 2 + 3 + 4 + ...... + 16) =
= 25 * 16 * (16 + 1) / 2 =
= 25 * 8 * 15 =
= 200 * 15 =
= 3000
g)
10 + 20 + 30 + 40 +....+ 480 =
= 10(1 + 2 + 3+ +4 + ......+ 48) =
= 10 * 48 * (48 + 1) / 2 =
= 10 * 24 * 49 = 10 * 1176 =
= 11760