When the same constant is added to the numbers 60, 120, and 160, a three-term geometric sequence arises. What is the common ratio of the resulting sequence?
In a geometric progression the ratio between succeeding numbers is the same.
Therefore 120 + x 160 + x
–––––– = ––––––
60 + x 120 + x
Cross multiplying (x + 120)(x + 120) = (x + 60)(x + 160)
x2 + 240x +14,400 = x2 + 220x + 9600
Subtract x2 from both sides 240x + 14,400 = 220x + 9600
Subtract 220x from both sides 20x + 14,400 = 9600
Subtract 10,000 from both sides 20x = – 4800
x = – 240
Plugging – 240
into the first ratio – 120
––––––– = 0.6666 • • •
– 180
Plugging – 240
into the second ratio – 80
––––––– = 0.6666 • • •
– 120
The common ratio, after
adding the constant, is 0.6666 • • • (or 2/3 if you prefer)
.