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Algebra Difficulty 5.2 AIME, harder Find the answer

Example 4 Suppose
x=11+11+11++1+11+ x=\frac{1}{1+\frac{1}{1+\frac{1}{1+\cdots}+\frac{1+1}{1+\cdots}}}

with 1000 horizontal fraction bars. Is x2+x>1x^{2}+x>1 always true? (1990, Wisconsin Talent Search)

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

Solution

Let xnx_{n} represent a number in the form of xx with nn fraction lines. Then, we have xn+1=11+xnx_{n+1}=\frac{1}{1+x_{n}}.
It is easy to find x1=12,x2=23,x3=35,x4=58x_{1}=\frac{1}{2}, x_{2}=\frac{2}{3}, x_{3}=\frac{3}{5}, x_{4}=\frac{5}{8},
\cdots. Let yn=xn2+xny_{n}=x_{n}^{2}+x_{n}, then
y1=341,y3=24251$.Conjecture:when$n$isodd,$yn1$.Wewillprovethisconjecture. y_{1}=\frac{3}{4}1, y_{3}=\frac{24}{25}1\$. Conjecture: when \$n\$ is odd, \$y_{n}1\$. We will prove this conjecture.
yn+11=xn+12+xn+11=(11+xn)2+(11+xn)1=(11+xn)2[1+(1+xn)(1+xn)2]=(11+xn)2(xn2+xn1)=(11+xn)2(yn1),\begin{aligned} \because & y_{n+1}-1=x_{n+1}^{2}+x_{n+1}-1 \\ & =\left(\frac{1}{1+x_{n}}\right)^{2}+\left(\frac{1}{1+x_{n}}\right)-1 \\ & =\left(\frac{1}{1+x_{n}}\right)^{2}\left[1+\left(1+x_{n}\right)-\left(1+x_{n}\right)^{2}\right] \\ & =-\left(\frac{1}{1+x_{n}}\right)^{2}\left(x_{n}^{2}+x_{n}-1\right) \\ & =-\left(\frac{1}{1+x_{n}}\right)^{2}\left(y_{n}-1\right), \end{aligned}
$yn+11$hastheoppositesignof$yn1$,whichmeanstheconjectureistrue.Thus,$y1000>1$. \$\therefore y_{n+1}-1\$ has the opposite sign of \$y_{n}-1\$, which means the conjecture is true. Thus, \$y_{1000}>1\$.
x 2 +x>1\text{x 2 +x>1} {. }

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.