Skip to main content
OlympiadHQ

Browse · MathNet

Print

Mathematica competitions in Croatia

Croatia algebra

Problem

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 $a_5 = a_3 r^2 = \frac{2}{3} \cdot 9 = 6-2-\frac{2}{3}\frac{2}{3}26$.
Final answer
-2, -2/3, 2/3, 2, 6

Techniques

Sequences and SeriesSimple Equations