Answer:

All we have to do now is to apply the formula sn=n2(2a+(n−1)d)) to determine the sum of the sequence. Thus, the sum of the first fifteen terms in the arithmetic sequence is 975

a= -28 , d= -24 - - 28 = -24 +28 = d= 4

n= 15

Sn = n/2 { 2a +(n-1)d}

Sn= 15/2 { -56 + 14× 4 }

Sn = 15/2 { -56 +56 }

Sn = 15/2 ×0

Sn = 0