Diketahui U2+U4=12 & U3+U5=16. Maka suku ke 7 adalah?

U2 + U4 = 12
=>a + b + a + 3b = 12
=> 2a + 4b = 12
U3 + U5 = 16
=> a + 2b + a + 4b = 16
=> 2a + 6b = 16
2a + 4b = 12
2a + 6b = 16
-------------------- -
-2b = -4
b = -4/-2
b = 2
2a + 4b = 12
2a + 4×2 = 12
2a + 8 = 12
2a = 12 - 8
2a = 4
a = 4/2
= 2

Sn = a + (n - 1) × b
S7 = 2 + (7 - 1) × 2
= 2 + 6 × 2
= 2 + 12
= 14
Jadi suku ke-7 adalah 14

pake cara coba-coba 2,4,6,8,10,12,14,16,18...
a. 4+8 = 12
b. 6+10=16
maka U7 = 14