Given a = -2, d = 4 and Sn = 160 find n.
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Sn=n/2[2a+(n-1)d]
160=n/2[-4+(n-1)4]
160=n/2[-4+4n-4]
160=n/2[-8+4n]
160*2=-8n+4n square
320=-8n+4n square
Therefore
4n square -8n-320=0
Divide by 4
N square -2n-80=0
n square-10n+8n-80=0
n(n-10)+8(n-10)=0
(n+8) (n-10)=0
n=-8 or n=10
n=10 (since n can not negative)
160=n/2[-4+(n-1)4]
160=n/2[-4+4n-4]
160=n/2[-8+4n]
160*2=-8n+4n square
320=-8n+4n square
Therefore
4n square -8n-320=0
Divide by 4
N square -2n-80=0
n square-10n+8n-80=0
n(n-10)+8(n-10)=0
(n+8) (n-10)=0
n=-8 or n=10
n=10 (since n can not negative)
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