Isosceles triangle construction given base and a side

Support this website at no cost to you learn how

Constructing an Isosceles Triangle (given base and one side)

Click here for a printable isosceles construction worksheet

This demonstration shows how to draw an isosceles triangle, using only a compass and straight edge. See "Introduction to Euclidean Constructions".

We start with two line segments AB and CD that define the lengths of the legs and the base of the triangle. The result is an isosceles triangle PQR. For more information, see Definition and Properties of an Isosceles Triangle.

Instructions Click on 'Next' to go through the construction one step at a time, or click on 'Run' to let it run without stopping.

(If there is no image below, see support page.)

Step-by-step Instructions

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

Try it yourself

Click here for a printable isosceles construction worksheet containing two problems to try. When you get to the page, use the browser print command to print as many as you wish. The printed output is not copyright.

Other constructions