Diketahui. S10 = 530. U7 =68. Ditnyakan u2

U 7 = a + 6b
a + 6b = 68
a = 68 - 6b ••••

Sn = n/2 ( 2a + ( n - 1 ) b )
S10 = 10/2 ( 2 . 68 - 6b + ( 10 - 1 ) b )
530 = 5 ( 136 - 6b + 9b )
530 = 5 ( 136 + 3b )
530 = 680 + 15b
-150 = 15b
-150/15 = b
b = -10

a = 68 - 6b
a = 68 - 6 (-10)
a = 128

Maka,
U2 = a + b
•••• = 128 + (-10)
•••• = 118