diketahui suku ke-5 dan suku ke-8 barisan aritmatika masing-masing 16 dan 25 jumlah 22 suku pertama adalah

Un = a + ( n - 1 ) b

U5 = a + 4b = 16
U8 = a + 7b = 25
------------------------(-)
- 3b = - 9
b = 3

a + 4b = 16
a + 4(3) = 16
a + 12 = 16
a = 16 - 12
a = 4

Sn = ½ • n ( 2a + ( n - 1 ) b )
S22 = ½ • 22 ( 2(4) + ( 22 - 1)3)
= 11 ( 8 + 21(3))
= 11 ( 8 + 63 )
= 11 × 71
= 781

U8= a+7b =25
U5= a+4b =16
sistem eliminasi
3b =9
b =9/3
b =3

a+4b =16
a+4(3)= 16
a+12 = 16
a = 16-12
a = 4

S22= ½n [2a + 21b]
=22/2 [2(4)+21(3)]
=11[8+63]
=11(71)
=781

Semoga bermanfaat^^