tolong aku biar cara hitung dulu

18) Barisan aritmetika dengan a = 2, b = 3
• Un = a + (n-1) b
U10 = 2 + (10-1) 3
= 2 + (9×3)
= 2+27
U10 = 29
• Sn = n/2 (a + Un)
S10 = 10/2 (2 + 29)
= 5(31)
= 155 (B)

19) a = 5, U6 = 15
• Un = a+(n-1) b
= 5 + (6-1) b = 15
= (6-1) b = 15-5
= (6-1) b = 10
= 5 b = 10
= b = 10÷5
= b = 2
• U9 = 5 + (9-1) 2
= 5+(8 × 2)
= 5+16
= 21
• S9 = 9/2 (5+21)
= 4,5 × 26
= 117 (C)

20) a = 50, U4 = 62
= 50 + (4-1) b = 62
= 50 + 3b = 62
= 3b = 62-50
= 3b = 12
= b = 12 ÷ 3
= b = 4
Rumus suku ke-n
Un = a + (n-1) b
Un = 50 + (n-1) 4
= 50 + 4n - 4
= 4n + 46 (B)