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Algebra Difficulty 5.5 AIME, harder Find the answer Croatia

Nikola is thinking of five numbers. The first number is 2-2, and the fifth number is 66. 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?

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let the five numbers be a1,a2,a3,a4,a5a_1, a_2, a_3, a_4, a_5.

Given:
- a1=2a_1 = -2
- a5=6a_5 = 6
- a1,a2,a3,a4a_1, a_2, a_3, a_4 are consecutive terms of an arithmetic progression (AP)
- a3,a4,a5a_3, a_4, a_5 are consecutive terms of a geometric progression (GP)

Let the common difference of the AP be dd.
Then:
a2=a1+d=2+d a_2 = a_1 + d = -2 + d
a3=a1+2d=2+2d a_3 = a_1 + 2d = -2 + 2d
a4=a1+3d=2+3d a_4 = a_1 + 3d = -2 + 3d

Let the common ratio of the GP be rr.
Then:
a4=a3r a_4 = a_3 r
a5=a3r2 a_5 = a_3 r^2
But a5=6a_5 = 6, a3=2+2da_3 = -2 + 2d, a4=2+3da_4 = -2 + 3d.

From the GP:
a4=a3r    2+3d=(2+2d)r a_4 = a_3 r \implies -2 + 3d = (-2 + 2d) r
a5=a3r2    6=(2+2d)r2 a_5 = a_3 r^2 \implies 6 = (-2 + 2d) r^2

From the second equation:
6=(2+2d)r2    r2=62+2d 6 = (-2 + 2d) r^2 \implies r^2 = \frac{6}{-2 + 2d}

From the first equation:
2+3d=(2+2d)r    r=2+3d2+2d -2 + 3d = (-2 + 2d) r \implies r = \frac{-2 + 3d}{-2 + 2d}

But r2=(2+3d2+2d)2=62+2dr^2 = \left(\frac{-2 + 3d}{-2 + 2d}\right)^2 = \frac{6}{-2 + 2d}

So:
(2+3d2+2d)2=62+2d \left(\frac{-2 + 3d}{-2 + 2d}\right)^2 = \frac{6}{-2 + 2d}
Multiply both sides by (2+2d)2(-2 + 2d)^2:
(2+3d)2=6(2+2d) (-2 + 3d)^2 = 6(-2 + 2d)
Expand:
(2+3d)2=412d+9d2 (-2 + 3d)^2 = 4 - 12d + 9d^2
So:
4 - 12d + 9d^2 = 6(-2 + 2d) = -12 + 12d
Bringalltermstooneside: Bring all terms to one side:
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 a5=a3r2=239=6a_5 = a_3 r^2 = \frac{2}{3} \cdot 9 = 6

Yes, it works.

Answer:

The numbers are 2-2, 23-\frac{2}{3}, 23\frac{2}{3}, 22, 66.

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