Question

If Sn = 1 + 6 + 11 + 16 + 21 + 26...... where an = (5n − 4), what would be the mathematical expression represented by Sk + ak + 1?

A) (k(5k − 3)/2)+ 5k + 1
B) (k(5k − 3)/2)+ 5k − 4
C) (k(5k − 3)/2)+ 5k + 2
D) (k(5k − 3)/2)+ 5k − 3
E) (k(5k − 3)/2)+ 5k − 8

Answer 1

as with any arithmetic progression,

Sk = k/2(a1 + ak)
= k/2(1+5k-4)
= k/2(5k-3)

Sk+ak+1 = k(5k-3)/2 + 5k-4+1

(D)

Don't forget your algebra I just because you're in pre-calc now.