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Algebra Difficulty 4.0 AMC 10/12 Find the answer

Consider A=log(2013+log(2012+log(2011+log(+log(3+log2)))))A = \log (2013 + \log (2012 + \log (2011 + \log (\cdots + \log (3 + \log 2) \cdots )))). Which of the following intervals contains AA?

Pick one

Solution

Solution 1
Let f(x)=log(x+f(x1))f(x) = \log(x + f(x-1)) and f(2)=log(2)f(2) = \log(2), and from the problem description, A=f(2013)A = f(2013)
We can reason out an approximation, by ignoring the f(x1)f(x-1):
f0(x)logxf_{0}(x) \approx \log x
And a better approximation, by plugging in our first approximation for f(x1)f(x-1) in our original definition for f(x)f(x):
f1(x)log(x+log(x1))f_{1}(x) \approx \log(x + \log(x-1))
And an even better approximation:
f2(x)log(x+log(x1+log(x2)))f_{2}(x) \approx \log(x + \log(x-1 + \log(x-2)))
Continuing this pattern, obviously, will eventually terminate at fx1(x)f_{x-1}(x), in other words our original definition of f(x)f(x).
However, at x=2013x = 2013, going further than f1(x)f_{1}(x) will not distinguish between our answer choices. log(2012+log(2011))\log(2012 + \log(2011)) is nearly indistinguishable from log(2012)\log(2012).
So we take f1(x)f_{1}(x) and plug in.
f(2013)log(2013+log2012)f(2013) \approx \log(2013 + \log 2012)
Since 100020171000 2017, then log(2012+log(2011+))>4\log(2012+\log(2011+\cdots))>4.
So 2012+log(2011+)>100002012+\log(2011+\cdots)>10000.
Repeating, we then get 2011+log(2010+)>1079882011+\log(2010+\cdots)>10^{7988}.
This is clearly absurd (the RHS continues to grow more than exponentially for each iteration).
So, xx is not greater than 20172017.
So A2.A 2. We are looking for f(2013)f(2013). First we show
Lemma. For any integer n>2n>2, if n2.n 2. We start with a simple observation:
Lemma. For x,y>2x,y>2, log(x+log(y))2\log(x+\log(y)) 2, we have xyxy=(x1)(y1)1>0xy-x-y = (x-1)(y-1) - 1 > 0, so xyxlog(y)>xyxy>0xy - x-\log(y) > xy - x - y > 0.
It follows that log(z+log(x+log(y)))<log(x)+log(y)+log(z)\log(z+\log(x+\log(y))) < \log(x)+\log(y)+\log(z), and so on.
Thus f(2010)<log2+log3++log2010<log2010++log2010<20094=8036f(2010) < \log 2 + \log 3 + \cdots + \log 2010 < \log 2010 + \cdots + \log 2010 < 2009\cdot 4 = 8036.
Then f(2011)=log(2011+f(2010))<log(10047)<5f(2011) = \log(2011+f(2010)) < \log(10047) < 5.
It follows that f(2012)=log(2012+f(2011))<log(2017)<4f(2012) = \log(2012+f(2011)) < \log(2017) < 4.
Finally, we get f(2013)=log(2013+f(2012))<log(2017)f(2013) = \log(2013 + f(2012)) < \log(2017), which leaves us with only option (A)\boxed{\textbf{(A)}}.

Solution 5 (nonrigorous + abusing answer choices.)
Intuitively, you can notice that log(2012+log(2011+(log(2)))<log(2013+log(2012+(log(2))))\log(2012+\log(2011+\cdots(\log(2)\cdots))<\log(2013+\log(2012+\cdots(\log(2))\cdots)), therefore (by the answer choices) log(2012+log(2011+(log(2)))<log(2021)\log(2012+\log(2011+\cdots(\log(2)\cdots))<\log(2021). We can then say:
x=log(2013+log(2012+(log(2))))x=\log(2013+\log(2012+\cdots(\log(2))\cdots))
log(2013)<x<log(2013+log(2021))\log(2013)<x<\log(2013+\log(2021))
log(2013)<x<log(2013+4)\log(2013)<x<\log(2013+4)
log(2013)<x<log(2017)\log(2013)<x<\log(2017)
The only answer choice that is possible given this information is (A)\boxed{\textbf{(A)}}

Solution 6 (super quick)
Let f(x)=log(x+log((x1)+log((x2)++log2)))f(x) = \log(x + \log((x-1) + \log((x-2) + \ldots + \log 2 \ldots ))). From the answer choices, we see that 3<f(2013)<43 < f(2013) < 4. Since f(x)f(x) grows very slowly, we can assume 3<f(2012)<43 < f(2012) < 4. Therefore, f(2013)=log(2013+f(2012))(log2016,log2017)    (A)f(2013) = \log(2013 + f(2012)) \in (\log 2016, \log 2017) \implies \boxed{\textbf{(A)}}.
~rayfish

Solution 7 (Not rigorous but it works)
Start small from log(3+log2)log(3+log2). Because log2log2 is close to 00, the expression is close to log3log 3. This continues for the 1 digit numbers. However, when we get to log(11+log10)log(11+log10), log10=1log10=1 so the expression equals log12log12. We can round down how much each new log contributes to the expression as an approximate answer. There are 90 2 digit numbers, 900 3 digit numbers, and 1014 4 digit numbers that increase the expression. 1000<901+9002+10143<100001000< 90 \cdot 1 + 900 \cdot 2 + 1014 \cdot 3< 10000, so it increases between 3 and 4. The answer is (A)\boxed{\textbf{(A)}}.
~dragnin

Video Solution by Richard Rusczyk
https://artofproblemsolving.com/videos/amc/2013amc12a/360

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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.