Mathematics Education Department


Triangle Construction

By Gooyeon Kim

 

Notation:

The angles at vertices A, B, and C: A, B, and C, respectively

The sides of the triangle opposite vertices A, B, and C: a, b, and c, respectively

The medians: m(a), m(b), m(c)

The altitude: h(a), h(b), h(c)

The segments along an angle bisector to the opposite side: d(a), d(b), d(c)

The radius of the circumcircle: R

The radius of the incircle: r

 

  • Triangle, given two sides and the angle opposite one of the sides

    a, b, A

     

    1) center A:

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    First, copy the segment of length b (segment AC).

     

     

    Rotate the segment AC with the center point A and angle A given.

     

     

    Construct a circle at point A with radius of the length a, i.e. segment BC.

     

     

    We can have the point B from the previous construction of a circle. B is located on the intersection point of the circle and the segment resulted from the rotation.

    Now, draw a line from the point B to the point C.

     

     

    Then we get a triangle of given conditions:

     

     

    GSP Sketch

    GSP Script


    2) center C:

    Draw a circle with the center C and radius a.

     

     

     

    If B = B', that is there is only one point, the triangle becomes right triangle.

     

     

    GSP Sketch (contri2.gsp)

    GSP Script (scontri2.gss)


     

    b, c, h(a)

     

     

    Begin with copying h(a) and construct two parallel lines at both end points of h(a). Take A at one point of end points.

     

     

    Draw a circle with center A and radius b.

     

     

    Construct another circle centered A and radius c and mark the intersection points.

     

     

    Then you have two possible triangles ABC' and AB'C.

     

     

     

    GSP Sketch

    GSP Script

     


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