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Verified answer
1) Un = a + ( n - 1 ) bU4 = a + 3b = 14
U6 + U8 = a + 5b + a + 7b
46 = 2a + 12b
a + 3b = 14 |×2
2a + 12b = 46
maka
2a + 6b = 28
2a + 12b = 46
--------------------(-)
- 6b = - 18
b = 3
a + 3b = 14
a + 3(3) = 14
a + 9 = 14
a = 14 - 9
a = 5
Sn = ½ • n ( 2a + ( n - 1 ) b )
S12 = ½ • 12 ( 2•5 + ( 12 - 1 ) 3)
= 6 ( 10 + 33)
= 6 • 43
= 258
2) a = 1
r = 5/1 = 25/5 = 5
Un = a.r^n-1
= 1 • (5)^n-1
Un = 5^n-1