Evaluate 12 sigma n=3 20(0.5)^n-1
9.99
19.95
29.98
39.96

We have a geometric series in which:

a = 20

r = 1/2

Now, we know that: S2 = 20(1-r^2)/(1-r)  and S12 = 20(1-r^12)/1-r

Therefore:  sigma(3,12) = S12-S2  = 20[(1-r^12)/(1-r) - (1-r^2)/(1-r)]  =5115/512  = 9.99023438

Therefore, the answer is: 9.99

for any of you who need the translation here it is

step-by-step explanation:

find the angular size of the equatorial radius of the earth, if the horizontal parallax of the sun

s

step-by-step explanation: