Consider the series:

1+1/2+1/4+1/8....

How many terms of this series must be taken before the sum first exceeds 1999/1000?

What I did was first input 1999/1000 into the series equation, so

Sn= n/2 (2u1 + (n-1)d)

1999/1000 = n/2 (2 + (n-1)(1/2) )

1999/1000 = n (2+ (1/2)n - (1/2) )

0 = 1/2 n^2 +3/2 n - 1999/500

Then i used the quad formula and got

1.7, -4.7

... but its wrong lol =|

i'm not sure what im supposed to do