Question

In an A.P., given an = 4, d = 2, Sn=-14, find n and
a.
:​

Answer 1

an=a+(n−1)d

4=a+(n−1)2

4=a+2n−2

6=a+2n

∴a=6−2n...(1)

Sn=2n[2a+(n−1)d]

−14=2n[2(6−2n)+(n−1)2]

−28=n[12−4n+2n−2]

−28=12n−2n2−2n

−28=10n−2n2

−14=5n−n2

Solving the above equation, we get,

(n−7)(n+2)=0

n=7,−2

∴n=7

a=6−2n  From (1)

a=6−2(7)=−8

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