SOLUTION: Sn=1218 a=8 difference(b)=5 how can i find the value of n

SOLUTION: Sn=1218 a=8 difference(b)=5 how can i find the value of n

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Question 1027538: Sn=1218
a=8
difference(b)=5
how can i find the value of n
Answer by mananth(16946) (Show Source): You can put this solution on YOUR website!

5n^2+11n-2436=0
solve by formula method
a= 5 , b= 11 c= -2436

b^2-4ac= 121 + 48720
b^2-4ac= 48841

x1=( -11 + 221 )/ 10
x1= 21.00

x2=( -11 -221 ) / 10
x2= -23.20
Ignore negative value
n=21

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