What is the sum of the geometric sequence 1, 3, 9, if there are 12 terms? (5 points)

The sum of the 12 terms is 265720

Step-by-step explanation:

* Lets revise the geometric sequence

- There is a constant ratio between each two consecutive terms in the

 geometric sequence

- Ex:

# 5  ,  10  ,  20  ,  40  ,  80  ,  ………………………. (×2)

# 5000  ,  1000  ,  200  ,  40  ,  …………………………(÷5)

- The rule of the general term in the sequence is

 where a is the first term , r is the common ratio between each two

 consecutive terms and n is the position of the term

- The sum of first n terms of a geometric series is calculated from

* Lets solve the problem

∵ The geometric sequence is 1 , 3 , 9 , .............

∵

∴

∵ There are 12 terms

∴ n = 12

∵ The first term is 1

∴ a = 1

∴

* The sum of the 12 terms is 265720

second option

step-by-step explanation:

the measures of a triangle equal 180. use that to find x.

2x+3x+4x=180

9x=180

9x/9=180/9

x=20

now you know x, you know angle a is 40°, angle b is 60°, angle c is 80°.

because angle a is 40°, the first option doesn't work.

because angle b is 60°, the second option does work.

when you add angle a and b, you get 100°. complementary means it equals 90°, so the third option doesn't work.

when you add angle a and c, you get 120°, which is not 100.