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# How to Construct an Isosceles Triangle

In this Article: Given All Side Lengths Given Two Equal Sides and the Angle Between Them Given The Base and Adjacent Angles Given the Base and Altitude Community Q&A 5 References

An isosceles triangle is a triangle with two equal side lengths and two equal angles.  Sometimes you will need to draw an isosceles triangle given limited information. If you know the side lengths, base, and altitude, it is possible to do this with just a ruler and compass (or just a compass, if you are given line segments). Using a protractor, you can use information about angles to draw an isosceles triangle.

## Steps

### Method 1Given All Side Lengths

1. 1
Assess what you know. To use this method, you should know the length of the triangle’s base and the length of the two equal sides. You can also use this method if you are given line segments representing the base and sides instead of the measurements.

• For example, you might know that the base of a triangle is 8 cm, and its two equal sides are 6 cm, or you might be given two lines, one representing the base, and one representing the two sides.
2. 2
Draw the base. Use a ruler to make sure that your line is measured exactly. For example, if you know that the base is 8 cm long, use a sharp pencil and a ruler to draw a line exactly 8 cm long.

• If using a given line segment instead of a measurement, draw the base by setting the compass to the width of the provided base. Make an endpoint, then use the compass to draw the other endpoint. Connect the endpoints using a straightedge. 
3. 3
Set the compass. To do this, open the compass to the width of the equal side lengths.

If you are given the measurement, use a ruler. If you are given a line segment, set the compass so that it spans the length of the line.

• For example, if the side lengths are 6 cm, open the compass to this length. Or, if provided a line segment, set the compass to the segment’s length.
4. 4
Draw an arc above the base. To do this, place the tip of the compass on one of the base’s endpoints. Sweep the compass in the space above the base, drawing an arc.

• Make sure the arc passes at least halfway across the base.
5. 5
Draw an intersecting arc above the base. Without changing the width of the compass, place the tip on the other endpoint of the base. Draw an arc that intersects the first one.

6. 6
Draw the sides of the triangle. Use a ruler to draw lines connecting the point where the arcs intersect to either endpoint of the base. The resulting figure is an isosceles triangle. 

### Method 2Given Two Equal Sides and the Angle Between Them

1. 1
Assess what you know. To use this method, you need to know the length of the two equal sides, and the measurement of the angle between these two sides. You can also use this method if you are given a line segment representing the side length instead of the measurement.

• For example, you might know that the isosceles triangle has two equal sides of 7 cm, or you might be given a line segment representing the side length. You also know that the angle between the sides is 50 degrees.
2. 2
Draw the angle. Use a protractor to construct the angle of the given measurement. Ensure that each of its vectors is longer than the given side length.

• For example, you might need to draw a 50-degree angle. Since the sides of the triangle are 7 cm, the vectors should be a little longer than 7 cm long. You can use a ruler or your compass set to the appropriate length to measure.
3. 3
Set the compass. If you know the measurement of the side lengths, use a ruler to open the compass to that length. If you are given a line segment instead of a measurement, use it to set the compass to the appropriate width.

• For example, if you know that the side lengths are 7 cm, then use a ruler to open your compass 7 cm wide.
4. 4
Draw an arc. To do this, place the tip of the compass on the vertex of the angle (where the two vectors meet). Draw one long arc that intersects each vector of the angle. You can also draw two small arcs, each one intersecting one of the vectors.

5. 5
Draw the base. Using a straightedge, draw a line connecting the points where the arc intersects the two vectors. The resulting figure is an isosceles triangle.

### Method 3Given The Base and Adjacent Angles

1. 1
Assess what you know. To use this method, you need to know the length of the base, or you need to be provided with a line segment that represents the base. You also need to know the measurement of the two angles adjacent to the base. Remember that the two angles adjacent to the base of an isosceles triangle will be equal.



• For example, you might know that an isosceles triangle has a base measuring 9 cm, with two adjacent 45-degree angles.
2. 2
Draw the base. If you know the measurement of the base, use a ruler to draw it the appropriate length. Make sure to measure exactly, and to create a straight line.

• You can also draw the base by setting the compass to the same width as a provided line segment. Draw an endpoint. Make the other endpoint using the compass. Then use a straightedge to connect the two endpoints.
3. 3
Draw the first angle. Use a protractor to draw the angle on the left side of the base. The vector should pass a little more than halfway over the base, so that it will intersect with the other side of the triangle.

4. 4
Draw the second angle. Use a protractor to draw the angle on the right side of the base. Make sure the second vector intersects the first. Where the two lines intersect creates the apex of the triangle. The resulting figure is an isosceles triangle.

### Method 4Given the Base and Altitude

1. 1
Assess what you know. To use this method, you need to know the length of the triangle’s base, and the height, or altitude, of the triangle. You can also use this method if you are given line segments representing the base and altitude instead of the measurements.

• For example, you might have an isosceles triangle with a base of 5 cm and a height of 2.5 cm.
2. 2
Draw the base. If you know the measurement, use a ruler. For example, if you know that the base is 5 cm long, use a ruler to draw a line that is exactly 5 cm long.

• If using a line segment instead of a measurement, draw the base by setting the compass to the width of the base. Draw an endpoint. Use the compass to draw the second endpoint. Then, connect the endpoints using a straightedge. 
3. 3
Draw a line bisecting the base. This means a line that cuts the line in half. You can use a compass and the method described here . Draw the line at least as long as the triangle’s altitude.

• You can also use a ruler and a protractor to bisect the line. Divide the length of the base in half. Use the ruler to draw a midpoint. Then, use a protractor to draw a line at this midpoint that intersects the base at a 90-degree angle.
4. 4
Set the compass. If you know the measurement of the altitude, use a ruler to open the compass to this exact length (for example, 2.5 cm). If you are given a line segment, open the compass to the length of the provided line.

5. 5
Draw an arc across the altitude. Place the tip of the compass on the midpoint of the base. Draw an arc across the bisecting line. You need to draw the arc only on one side of the base.

6. 6
Draw the triangle. Connect the point where the altitude and arc intersect with either endpoint of the base. The resulting figure will be an isosceles triangle.

## Community Q&A

Search

• Question
How do I construct a right-angled triangle abc such that ab = bc and ac = 10 cm?
Donagan

Draw a line segment 10 cm in length, and label it AC. Then choose any convenient length for AB and BC, and proceed as shown in Method 1.
Thanks!

Yes
No

• Question
How do I construct triangle AB = 10 cm, AC = 5 cm, BC = 8 cm?
Donagan

Draw straight line segment AB 10 cm in length. Using a compass, from point A draw an arc of 5 cm radius. From point B draw an arc of 8 cm radius. The intersection of the two arcs is point C. Draw straight lines from A to C and from B to C. You have your triangle.
Thanks!

Yes
No

• Question
How do I construct a right isosceles triangle given perimeter?
Donagan

You don’t have enough information to do that.
Thanks!

Yes
No

• Question
If the base is 60 and the base angle is 45, what is the length of the two sides?
Donagan

Both base angles are 45°. Therefore the third angle is 90°. Drop an altitude from the 90° angle to the base. The altitude bisects the 90° angle. It also bisects the base and is perpendicular to it. The altitude forms two smaller isosceles right triangles, each of which has two 45° angles and two sides with lengths of 30 (half the base). Thus, each 45° angle in each smaller right triangle has an opposite side and an adjacent side of length 30 and a hypotenuse of x (the length you’re trying to find). The sine (and cosine) of each 45° angle is 0.707. Therefore, 0.707 = 30 / x. x = 30 / 0.707 = 42.4. That’s the length of each of the two equal sides of the big triangle.
Thanks!

Yes
No

• Question
How would you construct an isosceles right triangle if only given the hypotenuse?
wikiHow Contributor

If you know the length of the hypotenuse, you can find the length of the other two sides of the triangle using the Pythagorean theorem (a^2 + b^2 = c^2). However, since this is an isosceles triangle, the two sides will be the same length, so you will simplify the Pythagorean formula to x^2 + x^2 = c^2, or 2x^2 = c^2.

For example, if the hypotenuse is 12 cm, the formula will be 2x^2 = 12^2:
2x^2 = 12^2
2x^2 = 144
2x^2/2 = 144/2
x^2 = 72
sqrt*x^2 = sqrt*72
x = 8.48.
Since every triangle has 180 degrees, if it is a right triangle, the angle measurements are 90-45-45. So the triangle will have a hypotenuse of 12, two side lengths of about 8.5 cm, and two 45 degree angles.

Thanks!

Yes
No

• Question
How do I construct and isosceles triangle whose base is 3 cm and base angle is 45?
Donagan

Call the triangle ABC with base BC. Draw BC of length 3 cm. Using a protractor, from B draw a line 45° from BC. On the same side of BC, from C draw another line 45° from BC. The intersection of these two lines is point A.
Thanks!

Yes
No

• Question
How do I construct isosceles triangle ABC, having the difference of its hypotenuse and side equal to 20 mm?
Donagan

You don’t have enough information to construct the triangle. For one thing, you don’t know whether the hypotenuse is larger or smaller than the other sides.
Thanks!

Yes
No

• Question
How do I construct an isosceles triangle when the base & side are given?
Donagan

Draw the base. From each end of the base mark off an arc with a radius equal to the length of the other side(s). The intersection of the arcs is the third vertex of the triangle. Connect the vertices. That’s your triangle.
Thanks!

Yes
No

• Question
How do I construct a triangle given 100 mm as side and the height as 70?
Donagan

Draw a straight line 100 mm in length. Consider it the base of the triangle. At any point on the base construct a perpendicular line 70 mm in length. Draw straight lines from the far end of the 70 mm line to each end of the 100 mm line. That forms a triangle. If it’s to be an isosceles triangle, construct the perpendicular line at the midpoint of the base.
Thanks!

Yes
No

• Question
What are the properties of an isosceles triangle?
Donagan

An isosceles triangle is one containing two (and only two) equal sides. The angles opposite the equal sides are also equal. The altitude drawn to the base (the non-equal side) of an isosceles triangle bisects the angle from which it’s drawn. The altitude also bisects and is perpendicular to the base.
Thanks!

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No

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## References

1. http://www.regentsprep.org/regents/math/geometry/gp6/Lisosceles.htm
2. http://www.mathopenref.com/constisosceles.html
3. http://www.regentsprep.org/regents/math/geometry/gc1/lisosconstruct.htm
4. https://www.mathsisfun.com/triangle.html
5. http://www.mathopenref.com/constisosceles2.html

## Article Info

Categories: Geometry

In other languages:

Português:  Desenhar um Triângulo Isósceles

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