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Difference between revisions of "2001 AIME II Problems/Problem 7"

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===Solution 3===
 
===Solution 3===
  
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The radius of an incircle is $ r=A_t/semiperimeter
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The radius of an incircle is <math> r=A_t/semiperimeter </math>. The area of the triangle is equal to <math> (90)(120)/2 </math> or 5400 and the semiperimeter is equal to <math> (90+120+150)/2 </math> or 180. The radius therefore is equal to <math>5400/180</math> or 30. Thus using similar triangles the dimensions of the triangle circumscribing the circle with center <math>C_2</math> are equal to <math>120-2(30)</math> or <math> 60</math>, <math>1/2(90)</math> or <math>45</math>, and <math>1/2(150)</math> or <math>75</math>. The radius of the circle inscribed in this triangle with dimensions <math>45</math>x<math>60</math>x<math>75</math> is found using the formula mentioned at the very beginning. The radius of the incircle is equal to 15. Defining P as (0,0), C_2 is equal to (60+15,15) or (75,15). Also using similar triangles, the dimensions of the triangle circumscribing the circle with center <math>C_3</math> are equal to <math>90-2(30)</math>, <math>1/3(120)</math>, <math>1/3(150)</math> or <math> 30,40,50</math>. The radius of C_3 by using the formula mentioned at the beginning  is 10. Using P as (0,0), <math>C_3</math> is equal to (<math>10, 60+10)</math> or <math>(10,70)</math>. Using the distance formula, the distance between <math>C_2</math> and <math>C_3</math>: <math>\sqrt{(75-10)^2 +(15-70)^2}</math> this equals <math>\sqrt{7250}</math> or <math>\sqrt{(725)(10)}</math>, thus n is <math>725</math>.
  
 
== See also ==
 
== See also ==

Revision as of 22:16, 2 October 2017

Problem

Let $\triangle{PQR}$ be a right triangle with $PQ = 90$, $PR = 120$, and $QR = 150$. Let $C_{1}$ be the inscribed circle. Construct $\overline{ST}$ with $S$ on $\overline{PR}$ and $T$ on $\overline{QR}$, such that $\overline{ST}$ is perpendicular to $\overline{PR}$ and tangent to $C_{1}$. Construct $\overline{UV}$ with $U$ on $\overline{PQ}$ and $V$ on $\overline{QR}$ such that $\overline{UV}$ is perpendicular to $\overline{PQ}$ and tangent to $C_{1}$. Let $C_{2}$ be the inscribed circle of $\triangle{RST}$ and $C_{3}$ the inscribed circle of $\triangle{QUV}$. The distance between the centers of $C_{2}$ and $C_{3}$ can be written as $\sqrt {10n}$. What is $n$?

Solution

Solution 1 (analytic)

[asy] pointpen = black; pathpen = black + linewidth(0.7); pair P = (0,0), Q = (90, 0), R = (0, 120), S=(0, 60), T=(45, 60), U = (60,0), V=(60, 40), O1 = (30,30), O2 = (15, 75), O3 = (70, 10); D(MP("P",P)--MP("Q",Q)--MP("R",R,W)--cycle); D(MP("S",S,W) -- MP("T",T,NE)); D(MP("U",U) -- MP("V",V,NE)); D(O2 -- O3, rgb(0.2,0.5,0.2)+ linewidth(0.7) + linetype("4 4")); D(CR(D(O1), 30)); D(CR(D(O2), 15)); D(CR(D(O3), 10)); [/asy]

Let $P = (0,0)$ be at the origin. Using the formula $A = rs$ on $\triangle PQR$, where $r_{1}$ is the inradius (similarly define $r_2, r_3$ to be the radii of $C_2, C_3$), $s = \frac{PQ + QR + RP}{2} = 180$ is the semiperimeter, and $A = \frac 12 bh = 5400$ is the area, we find $r_{1} = \frac As = 30$. Or, the inradius could be directly by using the formula $\frac{a+b-c}{2}$, where $a$ and $b$ are the legs of the right triangle and $c$ is the hypotenuse. (This formula should be used only for right triangles.) Thus $ST, UV$ lie respectively on the lines $y = 60, x = 60$, and so $RS = 60, UQ = 30$.

Note that $\triangle PQR \sim \triangle STR \sim \triangle UQV$. Since the ratio of corresponding lengths of similar figures are the same, we have

\[\frac{r_{1}}{PR} = \frac{r_{2}}{RS} \Longrightarrow r_{2} = 15\ \text{and} \ \frac{r_{1}}{PQ} = \frac{r_{3}}{UQ} \Longrightarrow r_{3} = 10.\]

Let the centers of $\odot C_2, C_3$ be $O_2 = (0 + r_{2}, 60 + r_{2}) = (15, 75), O_3 = (60 + r_{3}, 0 + r_{3}) = (70,10)$, respectively; then by the distance formula we have $O_2O_3 = \sqrt{55^2 + 65^2} = \sqrt{10 \cdot 725}$. Therefore, the answer is $n = \boxed{725}$.

Solution 2 (synthetic)

[asy] pointpen = black; pathpen = black + linewidth(0.7); pair P = (0,0), Q = (90, 0), R = (0, 120), S=(0, 60), T=(45, 60), U = (60,0), V=(60, 40), O1 = (30,30), O2 = (15, 75), O3 = (70, 10); D(MP("P",P)--MP("Q",Q)--MP("R",R,W)--cycle); D(MP("S",S,W) -- MP("T",T,NE)); D(MP("U",U) -- MP("V",V,NE)); D(O2 -- O3, rgb(0.2,0.5,0.2)+ linewidth(0.7) + linetype("4 4")); D(CR(D(O1), 30)); D(CR(D(O2), 15)); D(CR(D(O3), 10)); pair A2 = IP(incircle(R,S,T), Q--R), A3 = IP(incircle(Q,U,V), Q--R); D(D(MP("A_2",A2,NE)) -- O2, linetype("4 4")+linewidth(0.6)); D(D(MP("A_3",A3,NE)) -- O3 -- foot(O3, A2, O2), linetype("4 4")+linewidth(0.6)); [/asy]

We compute $r_1 = 30, r_2 = 15, r_3 = 10$ as above. Let $A_1, A_2, A_3$ respectively the points of tangency of $C_1, C_2, C_3$ with $QR$.

By the Two Tangent Theorem, we find that $A_{1}Q = 60$, $A_{1}R = 90$. Using the similar triangles, $RA_{2} = 45$, $QA_{3} = 20$, so $A_{2}A_{3} = QR - RA_2 - QA_3 = 85$. Thus $(O_{2}O_{3})^{2} = (15 - 10)^{2} + (85)^{2} = 7250\implies n=\boxed{725}$.


Solution 3

The radius of an incircle is $r=A_t/semiperimeter$. The area of the triangle is equal to $(90)(120)/2$ or 5400 and the semiperimeter is equal to $(90+120+150)/2$ or 180. The radius therefore is equal to $5400/180$ or 30. Thus using similar triangles the dimensions of the triangle circumscribing the circle with center $C_2$ are equal to $120-2(30)$ or $60$, $1/2(90)$ or $45$, and $1/2(150)$ or $75$. The radius of the circle inscribed in this triangle with dimensions $45$x$60$x$75$ is found using the formula mentioned at the very beginning. The radius of the incircle is equal to 15. Defining P as (0,0), C_2 is equal to (60+15,15) or (75,15). Also using similar triangles, the dimensions of the triangle circumscribing the circle with center $C_3$ are equal to $90-2(30)$, $1/3(120)$, $1/3(150)$ or $30,40,50$. The radius of C_3 by using the formula mentioned at the beginning is 10. Using P as (0,0), $C_3$ is equal to ($10, 60+10)$ or $(10,70)$. Using the distance formula, the distance between $C_2$ and $C_3$: $\sqrt{(75-10)^2 +(15-70)^2}$ this equals $\sqrt{7250}$ or $\sqrt{(725)(10)}$, thus n is $725$.

See also

2001 AIME II (Problems • Answer Key • Resources)
Preceded by
Problem 6 		Followed by
Problem 8
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