Difference between revisions of "2010 USAMO Problems/Problem 1"
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<math>O</math> is the midpoint of segment <math>AB</math>. | <math>O</math> is the midpoint of segment <math>AB</math>. | ||
− | ==Solution== | + | |
+ | |||
+ | |||
+ | ==Solution 0 (play like a God)== | ||
+ | |||
+ | Angles are directed mod 180 {don't worry}(take so that the solution can be applied to any configuration) Let the intersection of pq and rs is M. | ||
+ | |||
+ | <math>m\angle ARM=m\angle ZRS =m \angle ZYS =m\angle YZR = m\angle YZA = m \angle YXP = m \angle XYQ =m \angle XPQ = m \angle APM</math> This implies APRM is cyclic So: <math>m \angle PMR = m \angle PAR =m \angle XAZ = \frac{m\angle XOZ}{2}</math> and we are done. For any concern mail on harshsahu1098@gmail.com | ||
+ | |||
+ | ==Solution 1== | ||
Let <math>\alpha = \angle BAZ</math>, <math>\beta = \angle ABX</math>. | Let <math>\alpha = \angle BAZ</math>, <math>\beta = \angle ABX</math>. | ||
Since <math>XY</math> is a chord of the circle with diameter <math>AB</math>, | Since <math>XY</math> is a chord of the circle with diameter <math>AB</math>, | ||
Line 84: | Line 93: | ||
Similarly, <math>RY : YS = AY : YB</math>, and so <math>\angle YRS = \angle YAB = \alpha + \delta</math>. | Similarly, <math>RY : YS = AY : YB</math>, and so <math>\angle YRS = \angle YAB = \alpha + \delta</math>. | ||
− | Now <math>RY</math> is perpendicular to <math>AZ</math> so the direction <math>RY</math> is <math>\alpha</math> counterclockwise from the vertical, and since <math>\angle YRS = \alpha + \delta</math> we see that <math>SR</math> is <math>\delta</math> clockwise from the vertical. | + | Now <math>RY</math> is perpendicular to <math>AZ</math> so the direction <math>RY</math> is <math>\alpha</math> counterclockwise from the vertical, and since <math>\angle YRS = \alpha + \delta</math> we see that <math>SR</math> is <math>\delta</math> clockwise from the vertical. (Draw an actual vertical line segment if necessary.) |
Similarly, <math>QY</math> is perpendicular to <math>BX</math> so the direction <math>QY</math> is <math>\beta</math> clockwise from the vertical, and since <math>\angle YQP</math> is <math>\gamma + \beta</math> we see that <math>QY</math> is <math>\gamma</math> counterclockwise from the vertical. | Similarly, <math>QY</math> is perpendicular to <math>BX</math> so the direction <math>QY</math> is <math>\beta</math> clockwise from the vertical, and since <math>\angle YQP</math> is <math>\gamma + \beta</math> we see that <math>QY</math> is <math>\gamma</math> counterclockwise from the vertical. | ||
Line 100: | Line 109: | ||
below the point <math>Y</math>. | below the point <math>Y</math>. | ||
− | Since <math> | + | Since <math>YR = AY \sin(\delta)</math> and is inclined <math>\alpha</math> counterclockwise |
from the vertical, the point <math>S</math> is <math>AY \sin(\delta) \sin(\alpha)</math> | from the vertical, the point <math>S</math> is <math>AY \sin(\delta) \sin(\alpha)</math> | ||
horizontally to the right of <math>Y</math>. | horizontally to the right of <math>Y</math>. | ||
Line 121: | Line 130: | ||
Note that because <math>\angle{QPY}</math> and <math>\angle{YAB}</math> are both complementary to <math>\beta + \gamma</math>, they must be equal. Now, let <math>PQ</math> intersect diameter <math>AB</math> at <math>T'</math>. Then <math>PYT'A</math> is cyclic and so <math>\angle{YT'A} = 180^\circ - \angle{APY} = 90^\circ</math>. Hence <math>T'YSB</math> is cyclic as well, and so we deduce that <math>\angle{YST'} = \angle{YBT'} = 90^\circ - \alpha - \delta = \angle{YSR}.</math> Hence <math>S, R, T'</math> are collinear and so <math>T = T'</math>. This proves the Footnote. | Note that because <math>\angle{QPY}</math> and <math>\angle{YAB}</math> are both complementary to <math>\beta + \gamma</math>, they must be equal. Now, let <math>PQ</math> intersect diameter <math>AB</math> at <math>T'</math>. Then <math>PYT'A</math> is cyclic and so <math>\angle{YT'A} = 180^\circ - \angle{APY} = 90^\circ</math>. Hence <math>T'YSB</math> is cyclic as well, and so we deduce that <math>\angle{YST'} = \angle{YBT'} = 90^\circ - \alpha - \delta = \angle{YSR}.</math> Hence <math>S, R, T'</math> are collinear and so <math>T = T'</math>. This proves the Footnote. | ||
+ | |||
+ | ===Footnote to the Footnote to the Footnote=== | ||
+ | The Footnote's claim can be proved even more easily as follows. | ||
+ | |||
+ | Drop an altitude from <math>Y</math> to <math>AB</math> at point <math>T</math>. Notice that <math>P, Q, T</math> are collinear because they form the Simson line of <math>\triangle AXB</math> from <math>Y</math>. Also notice that <math>P, Q, T</math> are collinear because they form the Simson line of <math>\triangle AZB</math> from <math>Y</math>. Since <math>T</math> is at the diameter <math>AB</math>, lines <math>PQ</math> and <math>SR</math> must intersect at the diameter. | ||
+ | |||
+ | ===Footnote to the Fourth Power=== | ||
+ | There is another, simpler solution using Simson lines. Can you find it? | ||
+ | |||
+ | ==Operation Diagram== | ||
+ | Of course, as with any geometry problem, DRAW A HUGE DIAGRAM spanning at least one page. And label all your right angles, noting rectangles <math>PXQY</math> and <math>YSZR</math>. It looks like there are a couple of key angles we need to diagram. Let's take <math>\angle{ZAB} = \alpha, \angle{XBA} = \beta, \angle{YAZ} = \angle{YBZ} = \delta</math>. From there <math>\angle{XOZ}=180^\circ - \angle{XOA}-\angle{ZOB}=180-2(\beta + \alpha)</math>. | ||
+ | |||
+ | Move on to the part about the intersection of <math>PQ</math> and <math>RS</math>. Call the intersection <math>J</math>. Note that by Simson Lines from point <math>Y</math> to <math>\triangle{ABX}</math> and <math>\triangle{AZB}</math>, <math>YJ</math> is perpendicular to <math>AB</math> and <math>J</math> lies on <math>AB</math>. Immediately note that we are trying to show that <math>\angle{PJS} = 90 - \beta - \alpha</math>. | ||
+ | |||
+ | It suffices to show that referencing quadrilateral <math>QR~J</math>, where <math>~</math> represents the intersection of <math>XB, AZ</math>, we have reflex <math>\angle{Q~R} + \angle{BQJ} + \angle{ARJ} = 270 + \alpha + \beta</math>. Note that the reflex angle is <math>180^\circ + \angle{A~X} = 180^\circ + (90^\circ - \angle{XA*}) = 270^\circ - ((90 - \beta) - \alpha) = 180 ^\circ + \alpha + \beta</math>, therefore it suffices to show that <math>\angle{BQJ} + \angle{ARJ} = 90^\circ</math>. To make this proof more accessible, note that via (cyclic) rectangles <math>PXQY</math> and <math>YSZR</math>, it suffices to prove <math>\angle{YPJ} + \angle{YSJ} = 90^\circ</math>. | ||
+ | |||
+ | Note <math>\angle{YPJ} = \angle{YPQ} = \angle{YXQ} = \angle{YXB} = \angle{YAB} = \alpha + \delta</math>. | ||
+ | Note <math>\angle{YSJ} = \angle{YSR} = \angle{YZR} = \angle{YZA} = \angle{YBA} = \angle{YBX} + \angle{XBA} = ((90^\circ - \alpha) - \delta - \beta) + \beta = 90^\circ - \alpha - \delta</math>, which completes the proof. | ||
+ | |||
+ | ===Footnote to Operation Diagram=== | ||
+ | For reference/feasibility records: took expiLnCalc ~56 minutes (consecutively). | ||
+ | During the problem expiLnCalc realized that the inclusion of <math>\delta</math> was necessary when trying to show that <math>\angle{YSJ}+\angle{YPJ}=90^\circ</math>. Don't be afraid to attempt several different strategies, and always be humble! | ||
+ | |||
+ | ==Solution 2== | ||
+ | <asy> | ||
+ | currentpicture=new picture; | ||
+ | size(12cm); | ||
+ | pair O, A, B, X, Y, Z, P, Q, R, SS, T; | ||
+ | O=(0, 0); | ||
+ | A=(-1, 0); | ||
+ | B=(1, 0); | ||
+ | X=(Cos(144), Sin(144)); | ||
+ | Y=(Cos(105), Sin(105)); | ||
+ | Z=(Cos(27), Sin(27)); | ||
+ | P=foot(Y, A, X); | ||
+ | Q=foot(Y, B, X); | ||
+ | R=foot(Y, A, Z); | ||
+ | SS=foot(Y, B, Z); | ||
+ | T=foot(Y, A, B); | ||
+ | dot(O); dot(A); dot(B); dot(X); dot(Y); dot(Z); dot(P); dot(Q); dot(R); dot(SS); dot(T); | ||
+ | draw(arc(O, 1, 0, 180)); | ||
+ | draw(circumcircle(T, A, Y), dotted); | ||
+ | draw(circumcircle(T, B, Y), dotted); | ||
+ | draw(A -- B); | ||
+ | draw(Z -- O -- X -- A -- Z -- B -- X); | ||
+ | draw(A -- Y -- B); | ||
+ | draw(P -- T -- SS); | ||
+ | draw(P -- Y -- Q); draw(R -- Y -- SS); | ||
+ | draw(X -- P); draw(Z -- SS); | ||
+ | draw(Y -- T); | ||
+ | draw(rightanglemark(Y, T, B, 1.25)); | ||
+ | draw(rightanglemark(Y, P, A, 1.25)); | ||
+ | draw(rightanglemark(Y, Q, X, 1.25)); | ||
+ | draw(rightanglemark(Y, R, Z, 1.25)); | ||
+ | draw(rightanglemark(Y, SS, B, 1.25)); | ||
+ | draw(rightanglemark(A, X, B, 1.25)); | ||
+ | draw(rightanglemark(A, Y, B, 1.25)); | ||
+ | draw(rightanglemark(A, Z, B, 1.25)); | ||
+ | label("$O$", O, S); | ||
+ | label("$A$", A, SW); | ||
+ | label("$B$", B, SE); | ||
+ | label("$X$", X, (X-B)/length(X-B)); | ||
+ | label("$Y$", Y, Y); | ||
+ | label("$Z$", Z, (Z-A)/length(Z-A)); | ||
+ | label("$P$", P, (P-T)/length(P-T)); | ||
+ | label("$Q$", Q, SW); | ||
+ | label("$R$", R, SE); | ||
+ | label("$S$", SS, (SS-T)/length(SS-T)); | ||
+ | label("$T$", T, S); | ||
+ | </asy> | ||
+ | |||
+ | Let <math>T</math> be the foot of the perpendicular from <math>Y</math> to <math>\overline{AB}</math>, let <math>O</math> be the center of the semi-circle. | ||
+ | |||
+ | Since we have a semi-circle, if we were to reflect it over <math>\overline {AB}</math>, we would have a full circle, with <math>\triangle{AXB}</math> and <math>\triangle{AZB}</math> inscribed in it. Now, notice that <math>Y</math> is a point on that full circle, so we can say that <math>T</math> lies on the Simson Line <math>\overline{PQ}</math> from <math>Y</math> to <math>\triangle AXB</math> and that it also lies on the Simson line <math>\overline {RS}</math> from <math>Y</math> to <math>\triangle AZB</math>. Thus, <math>T</math> lies on two distinct lines in a plane, which means that <math>T=\overline{PQ}\cap\overline{RS}</math>. Therefore, it suffices to show that <math>\angle PTS=\tfrac{1}{2}\angle XOZ</math>. | ||
+ | |||
+ | Since <math>m\angle YTA + m\angle YPA = 90^\circ + 90^\circ = 180^\circ</math> and <math>m \angle YTB + m \angle YSB = 90^\circ + 90^\circ = 180^\circ</math>, we know that <math>TAPY</math> and <math>TBSY</math> are cyclic quadrilaterals. | ||
+ | |||
+ | We use this fact to get <cmath>\angle PTS=\angle PTY+\angle YTS=\angle PAY+\angle YBS=\angle XAY+\angle YBZ. \space \space (1)</cmath> \ | ||
+ | Now note that <math>\angle XAY</math> is the inscribed angle of minor arc <math>\overset{\huge\frown}{PY}</math>, and <math>\angle XOY</math> is the central angle of minor arc <math>\overset{\huge\frown}{AB}</math>, so <math>\angle XAY = \frac{\overset{\huge\frown}{PY}}{2} = \frac{\angle XOY}{2}</math>. Similarly, <math>\angle YBZ = \frac{\overset{\huge\frown}{YZ}}{2}=\frac{\angle YOZ}{2}</math>. Thus we can say <cmath>\angle XAY + \angle YBZ = \frac{\angle XOY}{2} + \frac{\angle YOZ}{2}=\frac{\angle XOY + \angle YOZ}{2} = \frac{\angle XOZ}{2}. \space \space (2)</cmath> | ||
+ | |||
+ | |||
+ | Combining statements <math>(1)</math> and <math>(2)</math>, we can say that <math>\angle PTS = \frac{\angle XOZ}{2}</math>, as desired. <math>\square</math> | ||
+ | |||
+ | ~thinker123 | ||
== See Also == | == See Also == |
Latest revision as of 02:15, 22 September 2024
Contents
[hide]Problem
Let be a convex pentagon inscribed in a semicircle of diameter . Denote by the feet of the perpendiculars from onto lines , respectively. Prove that the acute angle formed by lines and is half the size of , where is the midpoint of segment .
Solution 0 (play like a God)
Angles are directed mod 180 {don't worry}(take so that the solution can be applied to any configuration) Let the intersection of pq and rs is M.
This implies APRM is cyclic So: and we are done. For any concern mail on harshsahu1098@gmail.com
Solution 1
Let , . Since is a chord of the circle with diameter , . From the chord , we conclude .
Triangles and are both right-triangles, and share the angle , therefore they are similar, and so the ratio . Now by Thales' theorem the angles are all right-angles. Also, , being the fourth angle in a quadrilateral with 3 right-angles is again a right-angle. Therefore and . Similarly, , and so .
Now is perpendicular to so the direction is counterclockwise from the vertical, and since we see that is clockwise from the vertical. (Draw an actual vertical line segment if necessary.)
Similarly, is perpendicular to so the direction is clockwise from the vertical, and since is we see that is counterclockwise from the vertical.
Therefore the lines and intersect at an angle . Now by the central angle theorem and , and so , and we are done.
Note that is a quadrilateral whose angles sum to 360°; can you find a faster approach using this fact?
Footnote
We can prove a bit more. Namely, the extensions of the segments and meet at a point on the diameter that is vertically below the point .
Since and is inclined counterclockwise from the vertical, the point is horizontally to the right of .
Now , so is vertically above the diameter . Also, the segment is inclined clockwise from the vertical, so if we extend it down from towards the diameter it will meet the diameter at a point which is horizontally to the left of . This places the intersection point of and vertically below .
Similarly, and by symmetry the intersection point of and is directly below on , so the lines through and meet at a point on the diameter that is vertically below .
Footnote to the Footnote
The Footnote's claim is more easily proved as follows.
Note that because and are both complementary to , they must be equal. Now, let intersect diameter at . Then is cyclic and so . Hence is cyclic as well, and so we deduce that Hence are collinear and so . This proves the Footnote.
Footnote to the Footnote to the Footnote
The Footnote's claim can be proved even more easily as follows.
Drop an altitude from to at point . Notice that are collinear because they form the Simson line of from . Also notice that are collinear because they form the Simson line of from . Since is at the diameter , lines and must intersect at the diameter.
Footnote to the Fourth Power
There is another, simpler solution using Simson lines. Can you find it?
Operation Diagram
Of course, as with any geometry problem, DRAW A HUGE DIAGRAM spanning at least one page. And label all your right angles, noting rectangles and . It looks like there are a couple of key angles we need to diagram. Let's take . From there .
Move on to the part about the intersection of and . Call the intersection . Note that by Simson Lines from point to and , is perpendicular to and lies on . Immediately note that we are trying to show that .
It suffices to show that referencing quadrilateral , where represents the intersection of , we have reflex . Note that the reflex angle is , therefore it suffices to show that . To make this proof more accessible, note that via (cyclic) rectangles and , it suffices to prove .
Note . Note , which completes the proof.
Footnote to Operation Diagram
For reference/feasibility records: took expiLnCalc ~56 minutes (consecutively). During the problem expiLnCalc realized that the inclusion of was necessary when trying to show that . Don't be afraid to attempt several different strategies, and always be humble!
Solution 2
Let be the foot of the perpendicular from to , let be the center of the semi-circle.
Since we have a semi-circle, if we were to reflect it over , we would have a full circle, with and inscribed in it. Now, notice that is a point on that full circle, so we can say that lies on the Simson Line from to and that it also lies on the Simson line from to . Thus, lies on two distinct lines in a plane, which means that . Therefore, it suffices to show that .
Since and , we know that and are cyclic quadrilaterals.
We use this fact to get \ Now note that is the inscribed angle of minor arc , and is the central angle of minor arc , so . Similarly, . Thus we can say
Combining statements and , we can say that , as desired.
~thinker123
See Also
2010 USAMO (Problems • Resources) | ||
Preceded by First problem |
Followed by Problem 2 | |
1 • 2 • 3 • 4 • 5 • 6 | ||
All USAMO Problems and Solutions |
2010 USAJMO (Problems • Resources) | ||
Preceded by Problem 2 |
Followed by Problem 4 | |
1 • 2 • 3 • 4 • 5 • 6 | ||
All USAJMO Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.