## anonymous 3 years ago 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.

1. anonymous

use an=a1*r^(n-1)

2. anonymous

where an=48 and a1=7

3. anonymous

*448

4. anonymous

it is coming r^(n-1)=64

5. anonymous

then i used the sum formula Sn=$Sn =\frac{ a[r^n-1) }{ r-1 }$

6. anonymous

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

7. anonymous

i am stuck here

8. anonymous

where??tell me the second one

9. anonymous

$127=\frac{ r^n-1 }{ r-1 }$

10. anonymous

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

11. anonymous

The answer should come r=2 and n=7

12. anonymous

yeah hold on lemme write

13. anonymous

ok

14. anonymous

|dw:1356937412146:dw|

15. anonymous

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

16. anonymous

got it??

17. anonymous

yes

18. anonymous

good

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