Sum
In a G.P., the ratio between the sum of first three terms and that of the first six terms is 125 : 152. Find its common ratio.
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Solution
Let a be the first term and r be the common ratio of given G.P.
Now, sum of first three terms = S3 = `(a(r^3-1))/(r-1)`
Now, sum of first six terms = S6 = `(a(r^6-1))/(r-1)`
It is given that
`((a(r^3-1))/(r-1))/((a(r^6-1))/(r-1))=125/152`
`=> (r^3-1)/(r^6-1)=125/152`
`=>(r^3-1)/((r^3)^2-(1)^2)=125/152`
`=> (r^3-1)/((r^3-1)(r^3+1))=125/152`
`=>1/(r^3+1)=125/152`
`=>r^3+1=152/125`
`=>r^3=152/125-1=(152-125)/125=27/125`
`=>r=3/5`
Hence, the common ratio is `3/5`.
Concept: Geometric Progression - Finding Sum of Their First ‘N’ Terms
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