Question

Need help asap!!!!!!!!!

Answer 1

9514 1404 393

Answer:

  3420

Step-by-step explanation:

The first term is 20. The common ratio is -40/20 = -2. Fill these values into the formula and evaluate.

  $S_9=\dfrac{20-20\cdot(-2)^9}{1-(-2)}=\dfrac{20-20(-512)}{3}=\dfrac{10260}{3}\\\\ \boxed{S_9=3420}$

Answer 2

Answer:

-3400

Step-by-step explanation:

In general, a geometric sequence is given by:

a,ar,ar^2,⋯,ar^n−1

In the sequence 20, - 40 , 80,…

First term, a1 = 20

Common ratio, r = - 40 ÷ 20 = - 2

The nth term is = 9th

Sum of finite geometric series : Sn= ( a1 - a1r^n ) / ( 1 - r)

S9 = ( 20 - 20 x (- 2)^9 ) / ( 1 - ( - 2) )

S9 = ( 20 - 20 x - 512 ) / ( 1 + 2 )

S9 = ( 20 - 10240 ) / ( 3 )

S9 = ( - 10220 ) / 3

S9 = - 3400 Answer

Hope it's help you.... ^_^❤️