koli123able
In a G.P. , the first term is 7, the last term is 448, and the sum is 889. Find the common ratio and the number of terms.



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stgreen
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use an=a1*r^(n1)

stgreen
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where an=48 and a1=7

stgreen
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*448

koli123able
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it is coming r^(n1)=64

koli123able
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then i used the sum formula Sn=\[Sn =\frac{ a[r^n1) }{ r1 }\]

stgreen
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exactly...now you got two equations in two unknowns...so solve for r and n

koli123able
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i am stuck here

stgreen
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where??tell me the second one

koli123able
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\[127=\frac{ r^n1 }{ r1 }\]

stgreen
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r comes out to be 2.isn't it??

koli123able
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The answer should come r=2 and n=7

stgreen
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yeah hold on lemme write

koli123able
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ok

stgreen
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dw:1356937412146:dw

stgreen
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^from here you can get r=2....then put r=2 in first equation to get n

stgreen
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got it??

koli123able
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yes

stgreen
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good