(vi) given a=2, d=8, S =90, find n and a,;
vi given a = 8.4 = 62. S = 210, find n and d.
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Step-by-step explanation:
a=2,d=8,s
n
=90
s
n
=
2
n
[2a+(n−1)d]
90=
2
n
[4+(n−1)8]
180=n[8n−4]
180=8n
2
−4n
=8n
2
−4n−180=0
=2n
2
−n−45=0
=2n
2
−10n+9n−45=0
=2n(n−5)−9(n−5)
an=(2n−9)(n−5)
n=5
an =a+(n−1)d
an =2+(5−1)8
an=2+3+4
an =34
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