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

Algebra ->  Sequences-and-series -> SOLUTION: Sn=1218 a=8 difference(b)=5 how can i find the value of n      Log On


   

Question 1027538: Sn=1218
a=8
difference(b)=5
how can i find the value of n
Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
Sn+=%28n%2F2%29%282a%2B%28n-1%29d%29

1218=n%2F2%282%2A8%2B%28n-1%295%29

2436+=+n%2816%2B5n-5%29

2436+=+16n%2B5n%5E2-5n

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

b^2-4ac= 121 + 48720
b^2-4ac= 48841
%09sqrt%28%0948841%09%29=%09221%09
x=%28-b%2B-sqrt%28b%5E2-4ac%29%29%2F%282a%29
x1=%28-b%2Bsqrt%28b%5E2-4ac%29%29%2F%282a%29
x1=( -11 + 221 )/ 10
x1= 21.00
x2=%28-b-sqrt%28b%5E2-4ac%29%29%2F%282a%29
x2=( -11 -221 ) / 10
x2= -23.20
Ignore negative value
n=21