Sum of an arithmetic series: Sum = n( t1 + tn ) / 2
where n = number of terms t1 = first term = 1 tn = last term = 101
Since we don't know the number of terms, we can use this formula: tn = t1 + (n - 1)d
where d = common difference = 4
tn = t1 + (n - 1)d ---> 101 = 1 + (n - 1)4 ---> 101 = 1 + 4n - 4 ---> 101 = 4n - 3
---> 104 = 4n ---> n = 26
Sum = n( t1 + tn ) / 2 ---> Sum = 26( 1 + 101 ) / 2 ---> Sum = 1326