AlgebraDifficulty 5.0AIME, harderFind the answerUnited States
Problem:
Let f be a quadratic polynomial with real coefficients, and let g1, g2, g3, ... be a geometric progression of real numbers. Define an=f(n)+gn. Given that a1, a2, a3, a4, and a5 are equal to 1, 2, 3, 14, and 16, respectively, compute g1g2.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Solution:
We will use the method of finite differences. Define bn=an+3−3an+2+3an+1−an. Since f is quadratic, the third finite difference of f is zero. So, bn=gn+3−3gn+2+3gn+1−gn. Letting the common ratio of the geometric sequence be r, we get that bn=(r3−3r2+3r−1)gn. So, bn is a constant multiple of gn. Thus the ratio g1g2=b1b2. Computing b1=14−3⋅3+3⋅2−1=10 and b2=16−3⋅14+3⋅3−3⋅2=−19, we get
g1g2=b1b2=1019.
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Source: MathNet,
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