find Sn when u1=42, n=8, difference=6
That's the Sum of a Finite Arithmetic Series formula
to find Un we use the following formula
Un = u1 +(n-1)d
= 42 + (8-1)(6) = 42 + (7)(6) = 42+42 = 84
Then Sn = (8)(42+84) / 2 = (8)(126) / 2 = 504
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