Maths Olympiad Prep

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Algebra Difficulty 5.0 AIME, harder Find the answer United States

Problem:

Let ff be a quadratic polynomial with real coefficients, and let g1g_{1}, g2g_{2}, g3g_{3}, ... be a geometric progression of real numbers. Define an=f(n)+gna_{n} = f(n) + g_{n}. Given that a1a_{1}, a2a_{2}, a3a_{3}, a4a_{4}, and a5a_{5} are equal to 11, 22, 33, 1414, and 1616, respectively, compute g2g1\frac{g_{2}}{g_{1}}.

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+33an+2+3an+1anb_{n} = a_{n + 3} - 3a_{n + 2} + 3a_{n + 1} - a_{n}. Since ff is quadratic, the third finite difference of ff is zero. So, bn=gn+33gn+2+3gn+1gnb_{n} = g_{n + 3} - 3g_{n + 2} + 3g_{n + 1} - g_{n}. Letting the common ratio of the geometric sequence be rr, we get that bn=(r33r2+3r1)gnb_{n} = (r^{3} - 3r^{2} + 3r - 1)g_{n}. So, bnb_{n} is a constant multiple of gng_{n}. Thus the ratio g2g1=b2b1\frac{g_{2}}{g_{1}} = \frac{b_{2}}{b_{1}}. Computing b1=1433+321=10b_{1} = 14 - 3\cdot 3 + 3\cdot 2 - 1 = 10 and b2=16314+3332=19b_{2} = 16 - 3\cdot 14 + 3\cdot 3 - 3\cdot 2 = -19, we get

g2g1=b2b1=1910. \frac{g_{2}}{g_{1}} = \frac{b_{2}}{b_{1}} = \boxed{\frac{19}{10}}.

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