

Right triangle given one angle and one leg (LA)
Geometry construction using a compass and straightedge
This page shows how to construct a
right triangle that has one leg (L) and one angle (A) given.
It works in three steps:
 Copy the angle A. (See Copying an angle)
 Copy the length of the given leg onto the bottom angle leg (See Copying a segment)
 Erect a perpendicular from the end of the leg. (See Perpendicular to a line at a point)
Printable stepbystep instructions
The above animation is available as a
printable stepbystep instruction sheet, which can be used for making handouts
or when a computer is not available.
Proof

Argument 
Reason 
We first prove that ∆BCA is a right triangle 
1 
m∠BCA = 90° 
BC was constructed using the procedure in
Perpendicular to a line at a point.
See that page for proof.

2 
Therefore ∆BCA is a right triangle 
By definition of a right triangle, one angle must be 90° 
Now prove AC is congruent to the given leg 
3 
AC = the given leg 
AC was copied from the leg at the same compass width 
Now prove ∠BAC is the given angle A 
4 
m∠BAC = given m∠A 
Copied using the procedure in Copying an angle.
See that page for proof 
9 
∆BCA is a right triangle with the desired hypotenuse H and angle A 
From (2), (3), (4) 
 Q.E.D
Try it yourself
Click here for a printable worksheet containing two HA triangle construction problems.
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 pages on this site
Lines
Angles
Triangles
Right triangles
Triangle Centers
Circles, Arcs and Ellipses
Polygons
NonEuclidean constructions
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