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Geometry Difficulty 6.0 National olympiad Find the answer United States

A rectangle with side lengths 11 and 33, a square with side length 11, and a rectangle RR are inscribed inside a larger square as shown. The sum of all possible values for the area of RR can be written in the form mn\frac{m}{n}, where mm and nn are relatively prime positive integers. What is m+nm + n?
Figure 1

Pick one

Solution

Label the diagram as shown below, where OO, PP, and QQ are the feet of the perpendicular segments to the respective sides. Let x=AEx = AE and y=AFy = AF.

Figure 2

Because
FAEEOHIOHKCJFPG, \triangle FAE \cong \triangle EOH \cong \triangle IOH \cong \triangle KCJ \cong \triangle FPG,
it follows that EO=IO=KC=yEO = IO = KC = y and CJ=AE=xCJ = AE = x. Furthermore, JDI\triangle JDI is similar to all these triangles with scale factor 33, so ID=3xID = 3x and DJ=3yDJ = 3y. Therefore
3y+x=DC=DA=2y+4x, 3y + x = DC = DA = 2y + 4x,
so y=3xy = 3x. Applying the Pythagorean Theorem to AEF\triangle AEF yields x=110x = \frac{1}{\sqrt{10}}.

Now set PL=axPL = ax and LB=bxLB = bx for some positive real numbers aa and bb. Note that MQ=PG=y=3xMQ = PG = y = 3x, and because GPLLBM\triangle GPL \sim \triangle LBM, it follows that BM=13abxBM = \frac{1}{3}abx. Furthermore, NQKKCJ\triangle NQK \sim \triangle KCJ, and because NQ=PL=axNQ = PL = ax, it follows that QK=13axQK = \frac{1}{3}ax. Therefore
3x+13ax+3x+13abx=BC=AD=10x, 3x + \frac{1}{3}ax + 3x + \frac{1}{3}abx = BC = AD = 10x,
which simplifies to a(b+1)=12a(b + 1) = 12. Because PB=ABAP=6xPB = AB - AP = 6x, it follows that a+b=6a + b = 6. Solving these equations simultaneously for aa and bb shows that (a,b)(a, b) equals either (3,3)(3, 3) or (4,2)(4, 2).

Finally, observe that the area of RR is
LGLM=(ax)2+(3x)2(bx)2+(13abx)2=(a2+9)x2(b2+19a2b2)x2=x2a2+9b2+19a2b2=x2a2+9b2(1+19a2)=x2a2+9b1+19a2=x2b(a2+9)(1+19a2). \begin{aligned} LG \cdot LM &= \sqrt{(ax)^2 + (3x)^2} \cdot \sqrt{(bx)^2 + \left(\frac{1}{3}abx\right)^2} \\ &= \sqrt{(a^2 + 9)x^2} \cdot \sqrt{\left(b^2 + \frac{1}{9}a^2b^2\right)x^2} \\ &= x^2 \sqrt{a^2 + 9} \cdot \sqrt{b^2 + \frac{1}{9}a^2b^2} \\ &= x^2 \sqrt{a^2 + 9} \cdot \sqrt{b^2\left(1 + \frac{1}{9}a^2\right)} \\ &= x^2 \sqrt{a^2 + 9} \cdot b \sqrt{1 + \frac{1}{9}a^2} \\ &= x^2 b \sqrt{(a^2 + 9)\left(1 + \frac{1}{9}a^2\right)}. \end{aligned}
Since x=110x = \frac{1}{\sqrt{10}}, x2=110x^2 = \frac{1}{10}, so the area is
b10(a2+9)(1+19a2). \frac{b}{10} \sqrt{(a^2 + 9)\left(1 + \frac{1}{9}a^2\right)}.
When (a,b)=(3,3)(a, b) = (3, 3), this area equals 95\frac{9}{5}; and when (a,b)=(4,2)(a, b) = (4, 2), this area equals 53\frac{5}{3}. The sum of these two areas is 95+53=2715+2515=5215\frac{9}{5} + \frac{5}{3} = \frac{27}{15} + \frac{25}{15} = \frac{52}{15}, and the requested sum of numerator and denominator is 52+15=6752 + 15 = 67.

Figure 2

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.