a=2
d=8
Sn=90
find n
and an
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Answer:
Sn = (n/2)(2a+(n-1)d)
90 = (n/2)(4+(n-1)8)
180 = n(8n-4)
180 = 8n^2-4n
8n^2-4n-180 = 0
n=5
An = 2+4*8=34
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