MathDB

1957 AMC 12/AHSME

Part of AMC 12/AHSME

Subcontests

(49)

The Path of Point O'

In circle O O, G G is a moving point on diameter AB \overline{AB}. AA \overline{AA'} is drawn perpendicular to AB \overline{AB} and equal to AG \overline{AG}. BB \overline{BB'} is drawn perpendicular to AB \overline{AB}, on the same side of diameter AB \overline{AB} as AA \overline{AA'}, and equal to BG BG. Let O O' be the midpoint of AB \overline{A'B'}. Then, as G G moves from A A to B B, point O O': <spanclass=latexbold>(A)</span> moves on a straight line parallel to AB<spanclass=latexbold>(B)</span> remains stationary<spanclass=latexbold>(C)</span> moves on a straight line perpendicular to AB<spanclass=latexbold>(D)</span> moves in a small circle intersecting the given circle<spanclass=latexbold>(E)</span> follows a path which is neither a circle nor a straight line <span class='latex-bold'>(A)</span>\ \text{moves on a straight line parallel to }{AB}\qquad \\ <span class='latex-bold'>(B)</span>\ \text{remains stationary}\qquad \\ <span class='latex-bold'>(C)</span>\ \text{moves on a straight line perpendicular to }{AB}\qquad \\ <span class='latex-bold'>(D)</span>\ \text{moves in a small circle intersecting the given circle}\qquad \\ <span class='latex-bold'>(E)</span>\ \text{follows a path which is neither a circle nor a straight line}

Equilateral Triangle in a Circle

Let ABC ABC be an equilateral triangle inscribed in circle O O. M M is a point on arc BC BC. Lines AM \overline{AM}, BM \overline{BM}, and CM \overline{CM} are drawn. Then AM AM is: [asy]defaultpen(linewidth(.8pt)); unitsize(2cm);
pair O = origin; pair B = (1,0); pair C = dir(120); pair A = dir(240); pair M = dir(90 - 18);
draw(Circle(O,1)); draw(A--C--M--B--cycle); draw(B--C); draw(A--M); dot(O);
label("AA",A,SW); label("BB",B,E); label("MM",M,NE); label("CC",C,NW); label("OO",O,SE);[/asy]<spanclass=latexbold>(A)</span> equal to BM+CM<spanclass=latexbold>(B)</span> less than BM+CM<spanclass=latexbold>(C)</span> greater than BM+CM <span class='latex-bold'>(A)</span>\ \text{equal to }{BM + CM}\qquad <span class='latex-bold'>(B)</span>\ \text{less than }{BM + CM}\qquad <span class='latex-bold'>(C)</span>\ \text{greater than }{BM + CM}\qquad <spanclass=latexbold>(D)</span> equal, less than, or greater than BM+CM, depending upon the position of M <span class='latex-bold'>(D)</span>\ \text{equal, less than, or greater than }{BM + CM}\text{, depending upon the position of }{ {M}\qquad} <spanclass=latexbold>(E)</span> none of these <span class='latex-bold'>(E)</span>\ \text{none of these}

Bunch of Lines and Arcs in a Circle

In circle O O, the midpoint of radius OX OX is Q Q; at Q Q, ABXY \overline{AB} \perp \overline{XY}. The semi-circle with AB \overline{AB} as diameter intersects XY \overline{XY} in M M. Line AM \overline{AM} intersects circle O O in C C, and line BM \overline{BM} intersects circle O O in D D. Line AD \overline{AD} is drawn. Then, if the radius of circle O O is r r, AD AD is: [asy]defaultpen(linewidth(.8pt)); unitsize(2.5cm);
real m = 0; real b = 0;
pair O = origin; pair X = (-1,0); pair Y = (1,0); pair Q = midpoint(O--X); pair A = (Q.x, -1*sqrt(3)/2); pair B = (Q.x, -1*A.y); pair M = (Q.x + sqrt(3)/2,0);
m = (B.y - M.y)/(B.x - M.x); b = (B.y - m*B.x);
pair D = intersectionpoint(Circle(O,1),M--(1.5,1.5*m + b));
m = (A.y - M.y)/(A.x - M.x); b = (A.y - m*A.x);
pair C = intersectionpoint(Circle(O,1),M--(1.5,1.5*m + b));
draw(Circle(O,1)); draw(Arc(Q,sqrt(3)/2,-90,90)); draw(A--B); draw(X--Y); draw(B--D); draw(A--C); draw(A--D); dot(O);dot(M);
label("BB",B,NW); label("CC",C,NE); label("YY",Y,E); label("DD",D,SE); label("AA",A,SW); label("XX",X,W); label("QQ",Q,SW); label("OO",O,SW); label("MM",M,NE+2N);[/asy]<spanclass=latexbold>(A)</span> r2<spanclass=latexbold>(B)</span> r<spanclass=latexbold>(C)</span> not a side of an inscribed regular polygon<spanclass=latexbold>(D)</span> r32<spanclass=latexbold>(E)</span> r3 <span class='latex-bold'>(A)</span>\ r\sqrt {2} \qquad <span class='latex-bold'>(B)</span>\ r\qquad <span class='latex-bold'>(C)</span>\ \text{not a side of an inscribed regular polygon}\qquad <span class='latex-bold'>(D)</span>\ \frac {r\sqrt {3}}{2}\qquad <span class='latex-bold'>(E)</span>\ r\sqrt {3}

Equal Areas in a Triangle

From a point within a triangle, line segments are drawn to the vertices. A necessary and sufficient condition that the three triangles thus formed have equal areas is that the point be: <spanclass=latexbold>(A)</span> the center of the inscribed circle<spanclass=latexbold>(B)</span> the center of the circumscribed circle<spanclass=latexbold>(C)</span> such that the three angles fromed at the point each be 120<spanclass=latexbold>(D)</span> the intersection of the altitudes of the triangle<spanclass=latexbold>(E)</span> the intersection of the medians of the triangle <span class='latex-bold'>(A)</span>\ \text{the center of the inscribed circle} \qquad \\ <span class='latex-bold'>(B)</span>\ \text{the center of the circumscribed circle}\qquad \\ <span class='latex-bold'>(C)</span>\ \text{such that the three angles fromed at the point each be }{120^\circ}\qquad \\ <span class='latex-bold'>(D)</span>\ \text{the intersection of the altitudes of the triangle}\qquad \\ <span class='latex-bold'>(E)</span>\ \text{the intersection of the medians of the triangle}