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Upon a given line segment as a chord, to describe an arc of a circle in which a given angle can inscribed.
Given:  Segment AB, and RST
Required:  To construct on AB as a chord a segment of a circle in which R can be inscribed.
Procedure:

given angle             arc on given segment in which angle can be inscribed

  Label any point on ray SR P and any point on ray ST Q.
  Use the segment command under the construct menu and construct segment PQ.
  With segment AB as a base line, construct DAB = SPQ    and ABF = PQT  (help)
  Label the intersection of ray AD and ray BF point C.
  Use the segment command under the construct menu and construct segments AC and BC.
  Circumscribe a circle about ABC. (help)
  Arc ACB is the required arc.
Proof:
  ACB = RST    (If 2 s of one are = to 2 s of another , the 3rd s are =.)
  Any inscribed in arc ACB equals ACB.  (s inscribed in the same arc are =.)
Sketch:   
 
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describe an arc into which an angle can be inscribed

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