When the same constant is added to the numbers $60,$ $100,$ and $110,$ 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 100 + x 110 + x
–––––– = ––––––
60 + x 100 + x
Cross multiplying (x + 100)(x + 100) = (x + 60)(x + 110)
x2 + 200x +10,000 = x2 + 170x + 6600
Subtract x2 from both sides 200x + 10,000 = 170x + 6600
Subtract 170x from both sides 30x + 10,000 = 6600
Subtract 10,000 from both sides 30x = –3400
x = –113.333 • • •
Plugging –113.333
into the first ratio –13.333
––––––– = 0.24999 = 0.25
–53.333
Plugging –113.333
into the second ratio –3.333
––––––– = 0.24998 = 0.25
–13.333
Who'd a thunk it would really work.
The common ratio, after
adding the constant, is 0.25
.