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6.
b = U2 - U1 = 12 - 4 = 8
Un = a + (n-1)b = 4 + (n-1)8 = 4 + 8n - 8 = 8n - 4

7.
Un = 4n-2, n = 45
U45 = 4(45) - 2 = 178

8.
b = U2 - U1 = 13 - 6 = 7
Un = a + (n-1)b = 6 + (n-1)7 = 6 + 7n - 7 = 7n - 1

9.
a = 2, b = 6 - 2 = 4
U8 = a + 7b = 2 + 7x4 = 30

Sn = [n(a + Un)]/2 = [8(2 + 30)]/2 = 128

10.
U6 = a + 5b, b = 4
U6 = 25
25 = a + 5(4)
25 = a + 20
a = 5