Question

done

Answer 1

Yes you do!

Series starts at n=3, so first index is 3-1 = ????.

Yes?

Answer 2

wouldnt you add since it starts at 3 but goes to 12 so 1+2+3+4+5+6+7+8+9+10+11+12 like regular stiga

Answer 3

you can do a few things. one thing is write
\[ 0.5^{n-1}= 0.5^n \cdot 0.5^{-1} \]
0.5 to the -1 power means "flip" ½ to get 2
so the problem is
\[\sum_{n=3}^{12}20 \cdot 2 \cdot 0.5^n= 40 \sum_{n=3}^{12}0.5^n\]

Answer 4

I get 37.5 is that right

Answer 5

the first half I get 9.9

Answer 6

There is a formula for the sum from n=0 to 12. we could use that, and subtract off the sum from n=0 to 2

the answer is 5115/512 = 9.99023438

Answer 7

so I was right with 9.9 thank you very much