## koli123able 2 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. stgreen

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

2. stgreen

where an=48 and a1=7

3. stgreen

*448

4. koli123able

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

5. koli123able

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

6. stgreen

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

7. koli123able

i am stuck here

8. stgreen

where??tell me the second one

9. koli123able

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

10. stgreen

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

11. koli123able

The answer should come r=2 and n=7

12. stgreen

yeah hold on lemme write

13. koli123able

ok

14. stgreen

|dw:1356937412146:dw|

15. stgreen

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

16. stgreen

got it??

17. koli123able

yes

18. stgreen

good