AlgebraDifficulty 5.5AIME, harderFind the answerCroatia
Nikola is thinking of five numbers. The first number is −2, and the fifth number is 6. 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. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Let the five numbers be a1,a2,a3,a4,a5.
Given: - a1=−2 - a5=6 - a1,a2,a3,a4 are consecutive terms of an arithmetic progression (AP) - a3,a4,a5 are consecutive terms of a geometric progression (GP)
Let the common difference of the AP be d. Then: a2=a1+d=−2+d a3=a1+2d=−2+2d a4=a1+3d=−2+3d
Let the common ratio of the GP be r. Then: a4=a3r a5=a3r2 But a5=6, a3=−2+2d, a4=−2+3d.
From the GP: a4=a3r⟹−2+3d=(−2+2d)r a5=a3r2⟹6=(−2+2d)r2
From the second equation: 6=(−2+2d)r2⟹r2=−2+2d6
From the first equation: −2+3d=(−2+2d)r⟹r=−2+2d−2+3d
But r2=(−2+2d−2+3d)2=−2+2d6
So: (−2+2d−2+3d)2=−2+2d6 Multiply both sides by (−2+2d)2: (−2+3d)2=6(−2+2d) Expand: (−2+3d)2=4−12d+9d2 So: 4 - 12d + 9d^2 = 6(-2 + 2d) = -12 + 12d
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=2⋅924±242−4⋅9⋅16 d=1824±576−576 d=1824±0=1824=34 So d=34.
Now, compute the numbers: a1=−2 a2=−2+34=−32 a3=−2+2⋅34=−2+38=32 a4=−2+3⋅34=−2+4=2 a5=6 Check that a3,a4,a5 are in geometric progression:
Common ratio: r=a3a4=2/32=3 Check a5=a3r2=32⋅9=6
Yes, it works.
Answer:
The numbers are −2, −32, 32, 2, 6.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: MathNet,
licensed CC-BY-4.0.
Statement reproduced verbatim; metadata (topic, difficulty) added by this project.