Answers

2016-07-16T20:16:52+08:00
 S_{n}  a_{1}  a_{n} r / 1-r

750=  a_{1} - 400 ( 2 ) / 1 - 2

750 =  -a_{1} + 800

-50 =  -a_{1}

50 =  a_{1}

 a_{n}
0
Sorry accidentally sent it.. Here's the continuation.. an= a1 r^n-1 400= 50 (2^n-1) --> 8 = 2^n-1 --> n=4