sertakan caranya ya. terima kasih

Barisan dan deret

U8 = 19
a + 7b = 19

U15 + U18 =72
a + 14b + a + 17b = 72
2a + 31b = 72

a + 7b = 19 ( × 2)
2a + 31b = 72 (× 1)

2a + 14b = 38
2a + 31b = 72
___________ _
-17b = -34
b = -34/-17
b = 2

2a + 31b = 72
2a + 31(2) = 72
2a + 62 = 72
2a = 72 - 62
2a = 10
a = 5

Un = a + (n - 1)b
Un = 5 + (n - 1)2
Un = 5 + 2n - 2
Un = 2n + 3