The bisector of the angle BAC intersects the circle described about the triangle ABC at point D
The bisector of the angle BAC intersects the circle described about the triangle ABC at point D, so that BC = 4 DC = √5 find the radius of the circle and the cosine of the angle BAC.
From the center of the circle O we construct the radii OB, OD, OC.
We denote their length by R.
<BAD = <DAC (formed by the bisector AD).
The central angles are equal and resting on the same arcs:
<BOD = <DOC.
ΔBОС is isosceles, the segment OK is its bisector, and, consequently, the height and median.
The BC segment is perpendicular to OK.
KC = BK = BC / 2 = 2.
KD = √ (DC ^ 2 – KC ^ 2) = √ ((√5) ^ 2 – 2 ^ 2) = 1.
OC ^ 2 = KC ^ 2 + (OD – KD) ^ 2;
R ^ 2 = 2 ^ 2 + (R – 1) ^ 2
R ^ 2 = 4 + R ^ 2 – 2R +1;
2R = 5
R = 2.5.
OK = OD – KD = 2.5 -1 = 1.5;
cos (<KOC) = OK / OC = 1.5 / 2.5 = 0.6.
<KOC = 2 * <DAC (central and inscribed corners);
2 * <DAC = <BAC;
<KOC = <BAC;
cos (<BAC) = cos (<KOC) = 0.6.
Answer: R = 2.5; cos (<BAC) = 0.6.