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a = 9
b = 9
Un = a + (n - 1) b
99 = 9 + (n - 1) 9
99 = 9 + 9n - 9
99 = 9n
n = 99/9
n = 11
Sn = n/2 (a + Un)
S₁₁ = 11/2 (9 + 99)
= 11/2 × 108
= 594
Un = 99
b = 9
Un = a + (n-1)b
99 = 9 + (n-1)9
99 = 9 + 9n - 9
9n = 99
n = 11
Sn = n/2 (a + Un)
= 11/2 (9 + 99)
= 11/2 (108)
= 11(54)
= 594