How to construct triangles using three sides (SSS)
The original sides will have regular letters as labels (ex: ABC) the image will have the primes for
these letters (ex: A’B’C’)
(SSS triangle construction)
Given three sides (ex AB, BC, and CA)
(a)
Make the three sides as your originals, and make a ray on your image side.
Before going to the actual construction you must do the Triangle Inequality Test. To do
this test you must prove that the sum of any two sides is greater than the length of the
third side. To do the triangle inequality test on a ray add the two shorter segments
together and subtract the longest segment. If there is any length left label it “DIFF”
then proceed to the next step, otherwise state “DOES NOT PASS TI TEST”
(b)
(c)
(d)
(e)
Copy the longest segment onto the image ray labeling both endpoints (ex AB copies to A’B’)
Make an arc with a radius equal to one of the lengths of one of the two remaining original
sides to one endpoint on the image ray. Check that the one of the end points of the
original agrees with the letter of the image. (ex: for BC the arc MUST be centered at the B’
point on your image)
Make an arc with the radius equal to the length of the last remaining original side to the
other endpoint of the image(ex: for CA the arc MUST be centered at the A’ point on your
image)
Where the two arcs from steps (c) and (d) intersect is your third point C’. Label it C’ and
connect it to A’ and B’, and put tic marks on the sides to show congruence
Note: as you make the construction, label your new points (with the primes) and put tic marks on
the originals and image lengths that are congruent.
How to construct triangles using two sides and one angle (SAS)
or two angles and one side (ASA) as your originals parts
The original parts will have regular letters as labels (ex: ABC) the image will have the primes for
these letters (ex: A’B’C’)
(SAS triangle construction)
Given two sides (ex AB, BC) and one angle (ex: B)
(a) Make the two sides and the angle as your originals, and make a ray on your image side.
(b) Copy the angle onto the ray. Hint: Use the length of one of your original segments as your
measuring arc, picking one with an end point that agrees with the letter of your angle.
(c) After copying your angle. Label the new points for the angle, and the one side you used for
the measuring arc (you can pick either side of your angle). Put tic marks for the angles
and the side to show congruence to the originals
(d) Span your second side, and place the bulls-eye over the prime letter that agrees with one of
the original side’s two endpoints.
(e) The intersection of the arc in step (d) and the extended ray or the other side of the angle
is your third point. Label it, and connect it to the remaining point to complete the
triangle
(ASA triangle construction)
Given two angles (ex C, and B) and one angle (ex:CB)
(a) Make the two angles and one side as your originals, and make a ray on your image side.
Before going to the actual construction you must do the Triangle Sum Test. To do this
test you must add the two original angles together to show that they sum to less than 180
degrees. If the test passes (your two angles make less than a straight line) then
proceed to the next step, otherwise state “DOES NOT PASS ΔSUM TEST”
(b)
(c)
(d)
(e)
Copy the original side onto the image ray labeling both endpoints
Copy the both angles onto the two endpoints that agree with the letter for the original
angle. Hint: You can use the same measuring ray for both angles. Therefore you make a total
four arcs with the same radius (two on your originals, and two on the endpoints for your
image side.)
After copying your angles, put tic marks for the angles to show congruence to the originals.
Extend the sides to meet at your third vertex.
The intersection of the new sides in step (d) is your third point. Label it to complete the triangle.
How to construct triangles using Hypotenuse Leg (HL) as your
originals parts
The original parts will have regular letters as labels (ex: ABC) the image will have the primes for
these letters (ex: A’B’C’)
(HL triangle construction)
Given two sides one identified as a Hypotenuse and the other as a Leg and (ex Hyp: AB, Leg: BC)
(a) First you need to figure out which letter corresponds to your right angle. It is always the
one letter NOT on the hypotenuse. In the example the angle Hypotenuses is AB, so the missing
letter is the other letter in the leg – C
(b) On your image ray you need to pull a right angle from the point you found in step (a). When
doing this construction, use the leg length as your semicircle, then label your right angle
with a “box” and letter, and then the leg with the other endpoint and make the tic marks on
the leg to agree with that on your original. For the example: right angle is C’, and the leg
has B’ at the other endpoint.
(c) Span the hypotenuse, and place the bulls-eye over the endpoint of the leg in step (b). Make
an arc to pass thru the other side of your right angle. Note: you may need to extend that
other side long enough so it passes thru the arc. Where the arc passes the side label the
last point and make the tic marks on the hypotenuse to agree with the original
(Isosceles triangle construction)
There are numerous ways to construct isosceles triangles based on the original parts. Here are a
few examples
(1) Given the vertex angle (ex C) and one leg (ex:CB)
Copy the vertex angle using the leg length as the measuring arc. The base side is at the same
point that the spanning arc meets the measuring arc. (like SAS)
(2) Given a base angle (ex C) and the base(ex:CB)
Copy the base CB, then copy the base angle on both endpoints (like ASA)
(3) Given a base angle (ex C) and the one leg (ex:CB)
First find the vertex angle by finding the supplement to the twice the base angle. Measure of
vertex angle = 180 - 2mC. Then just follow the steps for (1)
(4) Given a vertex angle (ex C) and the base(ex:AB)
First find a base angle by bisecting the supplement to the vertex angle. Measure of base
angle = ½(180-mC). Then just follow the steps for (2)