U4 = 23 U7 = 5 Jumlah suku pertama dari deret aritmatika adalah....

a + 3b = 23
a + 6b = 5
_________ -
- 3b = 18
b = - 6
a + 3(-6) = 23
a - 18 = 23
a = 22 + 18
a = 41
U1 = a + 1 - 1b
= 41 - 6
= 35
Sn = n/2 (a + un)
= 1/2 ( 41 + 35)
= 1/2 (76)
= 38