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**Day one : Introduction to Circle**

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by Nami Youn

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Objective

This unit is for becoming familiar with the definitions
of the vocabulary associated with circles. Also, during this class, students
must be used to using Geometer's Sketchpad.
Let's investigate the definitions of terms about Circles

**1) Circle, Radius, Center**

A circle is the set of all points in a
plane that is the given same distance(radius)
from a given point(center).

The circle is the set of points that lie on the green curve.

**2) Chord**

A chord of a circle is a segment woth both endpoints
on the circle. A radius of a circle is a segment with one ednpoiont the
center of the circle and the other endpoint on the circle. A diameter of
a circle is a chord of the circle that the center of the circle lies on
the chord.

**3) Tangent, Secant**

A secant of a circle is a line S such that S meets
the circle at exactly two points A. Tangent of a circle is a line T such
that T meets the circle at exactly one point.

**4) Directed Arc and Central angle
of a directed arc**

A Directed arc of a circle is an arc together with an ordering of its
endpoints(B, C, and A). If The center of a circle is C and P is a directed
arc of the circle with ordered endpoints (A,B), the central angle of P
os the angle BCA.

**5) Inscribed angles**

If the points P, Q, R are on a circle, the angle
PQR is inscrived in the circle. The intercepted arc of the inscribed angle
PQR is the set of all points X on the circle such that ray(QX) is between
ray(QP) and ray(QR). It could be called the angel at circumference.

**5) Concentric circles**

Concentric circles are the circles that share the same center, but have
different radii.

**6) Semicircle, Minor angle and Major angle**

Major arc is an arc whose endpoints form an angle over 180 degrees with
the center of the circle: written CAB. The extra letter is used to distinguish
it from a minor arc: written BA.

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