MathDB
Finding the Length of a Partial Chord

Source:

March 8, 2009

Problem Statement

In a circle with center O O, AD AD is a diameter, ABC ABC is a chord, BO \equal{} 5, and ABO=CD=60 \angle ABO = \stackrel{\frown}{CD} = 60^{\circ}. Then the length of BC BC is:
[asy]size(200); defaultpen(linewidth(0.7)+fontsize(10)); pair O=origin, A=dir(35), C=dir(155), D=dir(215), B=intersectionpoint(dir(125)--O, A--C); draw(C--A--D^^B--O^^Circle(O,1)); pair point=O; label("AA", A, dir(point--A)); label("BB", B, dir(point--B)); label("CC", C, dir(point--C)); label("DD", D, dir(point--D)); label("OO", O, dir(305));
label("55", B--O, dir(O--B)*dir(90)); label("6060^\circ", dir(185), dir(185)); label("6060^\circ", B+0.05*dir(-25), dir(-25));[/asy]
(A)\ 3 \qquad (B)\ 3 \plus{} \sqrt3 \qquad (C)\ 5 \minus{} \frac{\sqrt3}{2} \qquad (D)\ 5 \qquad (E)\ \text{none of the above}