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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