help please!
Evaluate the summation of 20 times 0.5 to the n minus 1 power, from n equals 3 to 12..

9.99
19.95
29.98
39.96

Question

help please!
Evaluate the summation of 20 times 0.5 to the n minus 1 power, from n equals 3 to 12..

 9.99
 19.95
 29.98
 39.96

anonymous · Answer

$$\sum_{n=3}^{12} 20 \times 0.5^{n-1}$$

This is it, right?

anonymous · Answer

yes

anonymous · Answer

@larryboxaplenty

anonymous · Answer

I can't help you right now, sorry, g2g for a sec

someone else can help.  I know the answer for forget the shortcut

anonymous · Answer

can someone please help me

jhannybean · Answer

$$\large 20 \cdot \left(0.5^{n-1}\right)$$ start with plugging in $n=3$ and work your way up to $n=12$ by adding all the values.

jhannybean · Answer

$$\large 20\cdot (0.5^{3-1} + 0.5^{4-1}+0.5^{5-1}+...+0.5^{12-1})$$

welshfella · Answer

or you can use the formula for the sum of a geometric series

Sn = a . (1 - r^n)
             ------
               1 - r

welshfella · Answer

here a  =  20*0.5^2 ,  n = 11 and r = 0.5

anonymous · Answer

so would i get 9.99 as the answer

anonymous · Answer

@welshfella

welshfella · Answer

yes

anonymous · Answer

thank you

welshfella · Answer

yw