If r = 3 and the sum of 6 terms of the geometric series is 3640, what is the first term?

sum formula = s = a(1-r^n) / (1-r)

3640 = a (1 - 3^6) / (1-3)

3640 = a (1-729) / (-2)

Cross multiply

3640 * (-2) = a * (1-729)

- 7280 = a * (-728)

7280 = a * 728

Dividing by 728

a = 10 : Answer

Hope it will help :)

which expression represents the approximate length of bc?

step-by-step explanation:

a 4/6 = 9/12   not proportional

step-by-step explanation:

to find which are not proportional, we can use cross products

if it is true, they are proportional.   if it is not true, they are not proportional.

a 4/6 = 9/12

4* 12 = 6*9

48 = 54

not this is not proportional  

b 2/3 = 12/18  

2* 18 = 3 * 12

36 = 36

this is proportional

c 5/4 = 10/8  

5*8 = 4*10

40 = 40

this is proportional

d 7/5 = 35/25

7*25 = 5 * 35

175 = 175

is proportional