jumlah deretan aritmetika -21,-17,-13,-9 +... + 71 adalah

a = -21
b = -17 - (-21) = 4
Un = 71

Un = a + (n-1)b
71 = -21 + (n - 1)4
71 = -21 + 4n - 4
71 = 4n -25
4n = 71 + 25
4n = 96
n = 96/4
n = 24

Sn = n/2(a + Un)
S24 = 24/2(-21+ 71)
S24 = 12 x 50
S24 = 600

maka, jumlah deret aritmatika tsb adalah 600

a = -21
b = 4

un = a + (n-1)b
71 = -21 + (n-1) 4
71 = -21 + 4n - 4
71 = -25 + 4n
71+ 25 = 4n
96 = 4n
n = 96 / 4
n = 24

s24 = n/2 (2a+(n-1)b))
= 24/2 (2(-21) + (24-1)4)
= 12 (-42 + (23x4))
= 12( -42+ 92)
= 12 x 50
= 600