# Math

posted by .

Find a1 in a geometric series for which Sn=189,r=1/2,and an=3.

Sn=a1(1-r^n)/1-r
189=a1(1-1/2^3)/1-1/2
189=a1(-.5)/.5
189/-.25
a1=-756

• Math -

YOur third line is wrong

should be:

189 = a(1 - 1/8)/(1 - 1/2)
189 = a(7/8)/(1/2)
189 = a(7/4)
756 = 7a
108 = a

(check 108+54+27 = 189 !)

## Similar Questions

1. ### math. HOW DO I CONVERT A DECIMAL TO A FRACTION

how do i convert a decimal to fraction in SIMPLEST FORM?
2. ### chemistry

Temp - 650 Pressure - 760mm Hg mass - 0.927g volume - ?
3. ### chem

Consider a Carnot engine operating between the temperatures of 300. K and 189 K. Find the efficiency of this engine. Assume, now, that the isothermal step at 300. K is not reversible and the expansion takes place against a constant …
4. ### Geometry repost

"yes, except 20/36 / 21/x x= 189/5 = 37.8 seems to have a typo, no original equation" There was a typo, it should have been: 20/36 = 21/x x= 189/5 = 37.8 So is this correct?
5. ### MATH

1.32 + 0.987 + 3.07 = a-4.089 b-5.089 c-5.189 d-6.189 e-13.972 answer-5.089
6. ### college physics

A 0.384 kg puck, initially at rest on a horizon- tal, frictionless surface, is struck by a 0.189 kg puck moving initially along the x axis with a speed of 2.7 m/s. After the collision, the 0.189 kg puck has a speed of 1.7 m/s at an …
7. ### Algebra 2

5+(-5/2)+(5/4) Sn=189 Find n for the given sum if the geometric series
8. ### Math

Whars 189.80. Times 6.25% less 33 1/3 % Is it 189.80 + 11.86 = 201.66 - 67.22 = \$134..44
9. ### Chemistry

A chemist wants to make 150 mL of a 1.26 M solution of carbonic acid to be used in the lab. How many grams of H2CO3 does the chemist need to make this solution?
10. ### Math

You shop for bikes and find 7 types priced at 119 139 149 179 189 199 and 209. you want to buy the 189 bike would you use the mean median and mode to convince your parents that this is a reasonable price for a bike?

More Similar Questions