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2014-10-11T15:41:33+03:00
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