Diketahui suku ke-7 dan suku ke-10 suatu barisan aritmatika masing-masing 103 dan 88 jumlah 24 suku pertama adalah

Un = bn + a - b
U7 = 7b + a - b
103= 6b + a

Un = bn + a - b
U10 = 10b + a - b
88   = 9b + a

103 = 6b + a
88   = 9b + a
-------------------- -
  15 = -3b
    b = 15 / -3
    b = -5

88 = 9b + a
88 = 9(-5) + a
88 = -45 + a
 a  = 88 + 45
 a  = 133

Sn = n/2 (2a + (n-1)b)
S24 = 24/2 ( 2.133 + ( 24-1 ) -5 )
       = 12 (266 + (-115) )
       = 12 x 151
       = 1.812

Langsung aja y

U7 = a + 6b = 103
U10 = a + 9b = 88
_______________-
-3b = 15
b = -5

a + 9b = 88
a = 88 - 9(-5)
a = 88 + 45
a = 133

S24 = 24/2 (2a + 23b)
S24 = 12 (2.133 + 23(-5)
S24 = 12 ( 266 - 115)
S24 = 12 ( 151)
S24 = 1.812

Semoga berguna +_+