What is 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + 12 + 9 + 6 + 3?
Expain how to calculate 75 + 72 + 69 + 66 + ... + 21 + 18 + 15 + 12 + 9 + 6 + 3
We have (75 - 3) / 3 + 1 = 72/3 + 1 = 24 + 1 = 25 terms
Sum =
[ first term + last term ] * ( number of terms) / 2 =
[ 75 + 3 ] * ( 25 / 2) =
( [ 78 ] / 2 ) * 25 =
39 * 25 =
975
