2010 AMC 12B Problem 24

Attempt Problem 24 of the 2010 AMC 12B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2010 AMC 12B solutions, or check the answer key.

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24.

The set of real numbers xx for which 1x2009+1x2010+1x20111 \begin{aligned} &\frac{1}{x-2009}+\frac{1}{x-2010} \\ &\quad {}+\frac{1}{x-2011}\ge1 \end{aligned} is the union of intervals of the form a<xb.a\lt x\le b. What is the sum of the lengths of these intervals?

1003335\dfrac{1003}{335}

1004335\dfrac{1004}{335}

33

403134\dfrac{403}{134}

20267\dfrac{202}{67}

Answer: C
Concepts:rational equationinequalityVieta’s Formulas
Difficulty rating: 2320
Small Hint:

The left side is decreasing on each interval between its vertical asymptotes, so each solution interval ends at a root of the equation =1=1

Big Hint:

The three right endpoints are the roots of a cubic; sum them with Vieta

Solution:

Let f(x)f(x) be the left-hand side. On each interval between consecutive asymptotes 2009,2010,2011,2009, 2010, 2011, the function ff is decreasing, and f<1f\lt1 for all x<2009.x\lt2009.

On each of (2009,2010),(2009,2010), (2010,2011),(2010,2011), and (2011,),(2011,\infty), the solution is the part from the left asymptote up to a value xix_i where f(xi)=1.f(x_i)=1. So the solution set consists of three intervals with left endpoints 2009,2010,20112009, 2010, 2011 and right endpoints x1,x2,x3.x_1, x_2, x_3.

The total length is (x12009)(x_1-2009) +(x22010)+(x_2-2010) +(x32011)+(x_3-2011) =x1+x2+x36030.=x_1+x_2+x_3-6030.

Clearing denominators in f(x)=1f(x)=1 gives x3(2009+2010+2011+3)x2+=0, \begin{aligned} &x^3 \\ &\quad \small{}-(2009+2010+2011+3)x^2 \\ &\quad {}+\cdots=0, \end{aligned} whose roots are x1,x2,x3.x_1, x_2, x_3. By Vieta, x1+x2+x3=6033,x_1+x_2+x_3=6033, so the sum of lengths is 60336030=3.6033-6030=3.

Thus, the correct answer is C.

Problem 23#23
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