help me

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help me

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a1 = 3; an = 99; d = 3

an = a1 + (n-1)d

99 = 3 + (n-1)3

99-3 = 3n-3

*eliminate both the negative three

99 = 3n

*divide both by three

n = 33

Sn = n/2 [a1+an]

S33 = 33/2 [1+99]

S33 = 33/2 [100]

S33 = 1650

answer: S33 = 1650