Solving ASA Triangles

"ASA" means "Angle, Side, Angle"

ASA Triangle

This means we are given two angles and a side between the angles.

 

To solve an ASA Triangle

 

Example 1

In this triangle we know:

  • angle A = 76°
  • angle B = 34°
  • and c = 9

 

It's easy to find angle C by using 'angles of a triangle add to 180°':

So C = 180° - 76° - 34° = 70°

 

We can now find side a by using The Law of Sines:

a/sinA = c/sin C

a/sin76° = 9/sin70°

a = (9 × sin76°)/sin70° = 9.29 to 2 decimal places.

 

Similarly we can find side b by using The Law of Sines:

b/sinB = c/sin C

b/sin34° = 9/sin70°

b = (9 × sin34°)/sin70° = 5.36 to 2 decimal places.

 

Now we have completely solved the triangle i.e. we have found all its angles and sides.

 

Example 2

This is also an ASA triangle.

First find angle X by using 'angles of a triangle add to 180°':

X = 180° - 87° - 42° = 51°

Now find side y by using The Law of Sines:

y/sinY = x/sin X

So y/sin(87°) = 18.9/sin(51°)

So y = (18.9 × sin(87°))/sin(51°) = 24.29 to 2 decimal places.

Similarly we can find z by using The Law of Sines:

z/sinZ = x/sin X

So z/sin(42°) = 18.9/sin(51°)

So a = (18.9 × sin(42°))/sin(51°) = 16.27 to 2 decimal places.