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Constructing an isosceles triangle given the base and one side

This is the step-by-step, printable version. If you PRINT this page, any ads will not be printed.

See also the animated version.

After doing this Your work should look like this

We start with two line segments AB and CD that define the lengths of the legs and the base of the triangle.

Geometry construction with compass and straightedge or ruler or ruler
1.  Draw a point P that will become one end of the base of the triangle. Geometry construction with compass and straightedge or ruler or ruler
2.  Place the point of the compasses on the point C and adjust the compasses' width to the desired length CD of the base of the finished triangle Geometry construction with compass and straightedge or ruler or ruler
3.  With the compasses' point on P, make an arc. Geometry construction with compass and straightedge or ruler or ruler
4.  Pick a point R anywhere on the arc. This will become the other end of the base of the triangle. Geometry construction with compass and straightedge or ruler or ruler
5.  Draw the base line PR. Geometry construction with compass and straightedge or ruler or ruler
6.  With the compasses' point on B, set its width to the desired side length - AB Geometry construction with compass and straightedge or ruler or ruler
7.  Without changing the compasses, make two intersecting arcs - one from P, the other from R to define the third vertex, Q, of the triangle. Geometry construction with compass and straightedge or ruler or ruler
8.  Draw the two side lines PQ and RQ Geometry construction with compass and straightedge or ruler or ruler
9.  Done. The triangle PQR is an isosceles triangle. Geometry construction with compass and straightedge or ruler or ruler

Other constructions pages on this site

Lines

Angles

Triangles

Right triangles

Triangle Centers

Circles, Arcs and Ellipses

Polygons

Non-Euclidean constructions