Question

The sum of first three terms of a finite geometric series is -7/10 and their product is -1/125. [Hint: Use a/r , a, and ar to represent the first three terms, respectively.] The three numbers are _____, _____, and _____.

Answer 1

-1/10, -1/5 , -2/5 or -2/5, -1/5 , -1/10

Explanation:

let a/r, a, ar be the first 3 terms of the geometric series

their product would be equal to a^3

a^3=-1/125

a=-1/5

Substitute a=-1/5 into the first 3 terms

-1/5r + -1/5+-r/5=-7/10

Multiply the terms such that they have a common denominator:

-1/5r +-r/5r + -r^2/5r = -3.5r/5r

Multiply both sides by 5r

-r^2-r-1=-3.5r

Add 3.5r to both sides and multiply the equation by 2

-2r^2 + 5r -2=0

Factorize the equation

(2r-1)(r-2)=0

r=0.5 or r-2

For the first three terms where r=0.5

-2/5, -1/5 , -1/10

For the first three terms where r=2

-1/10, -1/5 , -2/5