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U5 - U3 = (a + (5 - 1)b) - (a + (3 - 1)b)
= (a + 4b) - (a + 2b)
= a - a + 4b - 2b
= 2b
2b = 16 - 10
2b = 6
b = 6/2
b = 3
U3 = a + 2b
a + 2b = 10
a + 2(3) = 10
a + 6 = 10
a = 10 - 6
a = 4
Sn = n/2 (2a + (n - 1)b)
= 6/2 (2(4) + (6 - 1)3)
= 3 (8 + (5)(3))
= 3 (8 + 15)
= 3 (23)
= 69