Maths Olympiad Prep

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Algebra Difficulty 4.8 AIME Find the answer Italy

Problem:

Let xx be the smaller of the two solutions of the equation x24x+2=0x^{2}-4x+2=0. What are the first three digits after the decimal point in the (base 10) representation of the number
x+x2+x3++x2009? x+x^{2}+x^{3}+\cdots+x^{2009} ?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

The answer is 414. From the usual formula for solving quadratic equations, we get x=22x=2-\sqrt{2}. Now using the formula for the sum of a geometric progression, we have
x+x2+x3++x2009=x(1+x+x2++x2008)=x1x20091x=x1xx20101x. \begin{aligned} x+x^{2}+x^{3}+\cdots+x^{2009} & =x\left(1+x+x^{2}+\cdots+x^{2008}\right) \\ & =x \frac{1-x^{2009}}{1-x}=\frac{x}{1-x}-\frac{x^{2010}}{1-x} . \end{aligned}
The second term of this sum is very small: indeed, with very loose estimates, we obtain
(22)2010<(0.6)2010<((0.6)2)1005<121005<1(210)100<110100. (2-\sqrt{2})^{2010}<(0.6)^{2010}<\left((0.6)^{2}\right)^{1005}<\frac{1}{2^{1005}}<\frac{1}{\left(2^{10}\right)^{100}}<\frac{1}{10^{100}} .
On the other hand, the first term equals
x1x=2221=(22)(2+1)(21)(2+1)=21. \frac{x}{1-x}=\frac{2-\sqrt{2}}{\sqrt{2}-1}=\frac{(2-\sqrt{2})(\sqrt{2}+1)}{(\sqrt{2}-1)(\sqrt{2}+1)}=\frac{\sqrt{2}}{1} .
Therefore the value of the expression is a quantity that differs from 2\sqrt{2} by less than 1010010^{-100}; the first digits of its decimal expansion will therefore be the same as those of 2\sqrt{2}, that is 1.4141.414 \ldots.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.