Nikola is thinking of five numbers. The first number is , and the fifth number is . The first four numbers are consecutive terms of an arithmetic progression, and the last three numbers are consecutive terms of a geometric progression. What numbers is Nikola thinking of?
Solution
Let the five numbers be .
Given:
-
-
- are consecutive terms of an arithmetic progression (AP)
- are consecutive terms of a geometric progression (GP)
Let the common difference of the AP be .
Then:
Let the common ratio of the GP be .
Then:
But , , .
From the GP:
From the second equation:
From the first equation:
But
So:
Multiply both sides by :
Expand:
So:
4 - 12d + 9d^2 = 6(-2 + 2d) = -12 + 12d
4 - 12d + 9d^2 + 12 - 12d = 0
(4 + 12) + 9d^2 - 12d - 12d = 0
16 + 9d^2 - 24d = 0
9d^2 - 24d + 16 = 0
This is a quadratic equation in $d$.
Solve for $d$:
d = \frac{24 \pm \sqrt{24^2 - 4 \cdot 9 \cdot 16}}{2 \cdot 9}
d = \frac{24 \pm \sqrt{576 - 576}}{18}
d = \frac{24 \pm 0}{18} = \frac{24}{18} = \frac{4}{3}
So $d = \frac{4}{3}$.
Now, compute the numbers:
a_1 = -2
a_2 = -2 + \frac{4}{3} = -\frac{2}{3}
a_3 = -2 + 2 \cdot \frac{4}{3} = -2 + \frac{8}{3} = \frac{2}{3}
a_4 = -2 + 3 \cdot \frac{4}{3} = -2 + 4 = 2
a_5 = 6
Check that $a_3, a_4, a_5$ are in geometric progression:
Common ratio:
r = \frac{a_4}{a_3} = \frac{2}{2/3} = 3
Check
Yes, it works.
Answer:
The numbers are , , , , .