Overview of Section 2.1 Basic Properties of Arcs and Central and Inscribed Angles
Definitions
Circle
Radius
Secant
Chord
Diameter
Congruent circles -- iff equal radii.
Arc of a circle
Major Arc
Minor Arc
Central angle
Measure of the minor arc is the measure of the central angle.
Adjacent arcs
Congruent arcs
Propositions. Note, these are intuitive statements that could be proved, but since the proofs are omitted they are called 'propositions.'
Proposition 2.1: Two minor arc in the same circle or in congruent circles are congruent if and only if their central angles are congruent.
Proposition 2.2: In a circle, or in congruent circles, two arcs are congruent if and only if the corresponding chords are congruent.
Proposition 2.3: A diameter perpendicular to a chord bisects the chord and bisects each of the arcs determined by the chord
Proposition 2.4: If two circles intersect, the line through their centers is the perpendicular bisector of their common chord.
A proof was given for Proposition 2.4.
Tangent to a Circle
Tangent to a curve: A tangent at a point P on a curve . . . is a line containing P and having the same direction of the curve at P. If Q is another point on the curve, PQ is a secant. As Q gets close to P from either side, PQ approaches the tangent line to the curve at P. Note we could have a secant PQ that is also a tangent line.
Definition of a Tangent to a Circle: A tangent to a circle O at a point P on the circle is the line through P, perpendicular to the radius OP.
NOW SOLVE THIS 2.1
Prove: The tangent to the circle intersects the circle in exactly one point.
An Angle Formed by a Tangent and a Secant
This section discusses a tangent and secant having a common point P on the circle.
Theorem 2.1: The measure of the angle formed by a tangent and a chord (secant?) measures 1/2 of the intercept arc.
Now Solve This 2.2. Alternative Proof of Theorem 2.1 -- Open GSP File
The Inscribed Angle Theorem
Inscribe angle -- an angle with its vertex on the circle and whose sides lie along chords of the circle.
Problem 2.1 Find a relationship between the measure of an inscribed angle and the measure of its intercepted arc.
Solution. Use Theorem 2.2. For an inscribed angle, construct the tangent line t at the vertex.
Theorem 2.2: The Inscribed Angle Theorem
The measure of an inscribed angle equals half the measure of the intercepted arc.
Open GSP File Within the GSP File drag point A anywhere on the arc BAC
Corollary 2.1: In any circle, all of the inscribed angle intercepting the same arc are congruent.
Proof: By Theorem 2.2.
Theorem: 2.3: If the measure of angle A, an inscribed angle subtending chord BC of a circle, then the locus of all points (in the half plane determined by line BC and a point A on the circle) from which the chord BC is seen by the measure of angle A is the arc BAC.
Proof: We need to show that if P is not on the arc BAC but in the half plane as designated, then the measure of angle BPC ≠ the measure of angle BAC. Two cases: P inside the circle and P outside the circle
In either case the strategy is to construct a point Q on the circle to create an angle BQC which by Corollary 2.1 is congruent to angle BAC. In Case 1, we extend BP to intersect the circle at Q. In Case 2, we select the point Q where BP intersects the circle.
In each case by Corollary 2.1 the measure of the angle at Q is the same as the measure at A. Examine the triangle QPC in Case 1. The angle at P is the exterior angle of triangle QPC and so it is larger than the angle at Q. Therefore the measure of the angle BAC ≠ the measure of angle BPC. In somewhat the same way, examine the triangle PQC in Case 2. Angle BQC is an exterior angle and therefore has greater measure than the angle at P. So the measure of the angle at at A ≠ the measure of the angle at P.
NOW SOLVE THIS 2.3
1. Completion of the proof of Thm 2.3 in Case 2. We have completed that in the previous discussion.
2. Given a segment construct the locus of all the points in the plane as seen by an angle of (a) 45 degrees and (b) 30 degrees. We showed a construction in class for any given angle.
Use the script tool in the GSP file to complete the construction for a 45 degree (use a 45-90-45 triangle) and 30 degree (use a 30-60-90 triangle -- half of an equilateral triangle).
Corollary 2.2: Any angle inscribed in a semicircle is a right angle. Open GSP file.
Contrast this proof with the one we did for Problem 2.13.
Example 2.1 Open GSP file.
NOW SOLVE THIS 2.4.
1. Construct a 45 degree angle and use the information from example 2.1 to trisect the angle. Open the GSP file from Example 2.1 again. The triangle BOC is an isosceles right triangle. Therefore the angle BOC has been trisected by angle BOD. Bisect angle DBC to get the other two 15 degree trisections.
2. Trisect a 45 degree angle in a different way. Open GSP file for one alternative way.
Quadrilaterals Inscribed in a Circle
A polygon is inscribed in a circle if each vertex is on the circle. To say that a polygon is cyclic is the same as that it is inscribed, although a cyclic polygon is assumed to have the vertices named in order around the circle. To say that the circle is circumscribing the polygon is equivalent.
Every triangle is cyclic; it can be inscribed in a circle; it can be circumscribed by a circle.
When is a quadrilateral cyclic?
Precursor to Theorem 2.4: A necessary and sufficient condition for a quadrilateral to be cyclic is that the perpendicular bisectors of the sides are concurrent.
Theorem 2.4: A necessary and sufficient condition for a quadrilateral to be cyclic is that the sum of the measures of a pair of opposite angles is 180 degrees.
Necessary -- if the quadrilateral is inscribed in a circle then the sum of the measures of a pair of opposite sides is 180 degrees.
Proof: Take an inscribed quadrilateral and construct one of its diagonals.
This is what many school mathematics texts consider as the proof of this theorem. It is only the necessity condition proof
The quadrilateral is divided into two triangles. The Angle at B is subtending arc ADC and the angle at D is subtending the arc ABC. The total of the two arc is 360 degrees and so the the sum of the measures is 180 degrees.
Sufficiency -- If the sum of the measures of opposite angles is 180 degrees, then the triangle is cyclic. That is, given the measures of angle G and angle E sum to 180 degrees.
Proof:
Assume EFGH is not cyclic and look for a contradiction. Take E to not be on the circle. Construct a point E' by extending side HE to the circle and construct E'F.
By the first part of the theorem, E'FGH has the sum of measures the angles at E' and and F to be 180 degrees. Consider the triangle E'FE. The angle at E is an exterior angle and therefore greater than either of the opposite interior angles. Therefore the angle at E is greater than the angle at E'. This is a contradiction since both of them can not sum with the measure at G to give 180.
In a similar manner, if E was outside of the circle we would build a contradiction and thereby complete the proof.
NOW SOLVE THIS 2.5
Two constructions and explorations to be done with GSP.
1. OPEN GSP File This exploration constructs an inscribed quadrilateral with perpendicular bisectors and then a perpendicular to one of the sides from the point of intersection of the diagonals. You are to conjecture and prove something about the intersection of this latter perpendicular with the opposite side.
2. Draw any quadrilateral ABCD (not necessarily cyclic) and construct the angle bisectors of the four angles. The intersections determine a new quadrilateral XYZW for exploration and conjecture. Open GSP File Run the Animate. A circle through points XY and W has been constructed. As the animation runs, it appears to always go through point Z. Conjecture: the quadrilateral XYZW will be cyclic. Proof?
