By "solving" I mean finding missing sides and angles.
If you know any 3 of the sides or angles ...
... you can find the other 3
|(Except for 3 angles,
need at least
one side to find how big the triangle is.)
Six Different Types
If you need to solve a triangle right now, then choose one of the six options below:
Which Sides or Angles do you know already? (Click on the image, or link)
|Three Angles||Two Angles and
a Side not between
|Two Angles and
a Side between
|Two Sides and
an Angle between
|Two Sides and
an Angle not between
... or read on to find out how you can become an expert triangle solver:
Your Solving Toolbox
Want to learn to solve triangles?
Imagine you are "The Solver" ... the one they ask for when a triangle needs solving!
In your solving toolbox (along with your pen, paper and calculator) you have these 3 equations:
A + B + C = 180°
When you know two angles you can find the third.
2. Law of Sines (the Sine Rule):
If there is an angle opposite a side, this equation will come to the rescue.
Note: angle A is opposite side a, B is opposite b, and C is opposite c.
3. Law of Cosines (the Cosine Rule):
This is the hardest to use (and remember) but it is sometimes needed to get you out of difficult situations.
It is an enhanced version of the Pythagoras Theorem that works on any triangle.
Six Different Types (More Detail)
There are SIX different types of puzzles you may need to solve. Get familiar with them:
This means we are given all three angles of a triangle, but no sides.
A AAA triangle is impossible to solve further since there are is nothing to show us size ... we know the shape but not how big it is.
You are going to need to know at least one side to proceed further. See Solving "AAA" Triangles .
This mean we are given two angles of a triangle and one side, which is not the side adjacent to the two given angles.
This means we are given two angles of a triangle and one side, which is the side adjacent to the two given angles.
This means we are given two sides and the included angle.
For this type of triangle, we must use The Law of Cosines first to calculate the third side of the triangle; then we can use The Law of Sines to find one of the other two angles, and finally use Angles of a Triangle to find the last angle. See Solving "SAS" Triangles .
This means we are given two sides and one angle that is not the included angle.
In this case, use The Law of Sines first to find either one of the other two angles, then use Angles of a Triangle to find the third angle, then The Law of Sines again to find the last remaining side. See Solving "SSA" Triangles .
This means we are given all three sides of a triangle, but no angles.
In this case, we have no choice. We must use The Law of Cosines first to find any one of the three angles, then we can use The Law of Sines (or use The Law of Cosines again) to find a second angle, and finally Angles of a Triangle to find the third angle. See Solving "SSS" Triangles .
Tips to Solving
Here is some simple advice:
- If the triangle has a right angle, then use it - that is usually much simpler.
- If the triangle has no right angle, then the type of triangle will determine whether we use The Law of Sines or The Law of Cosines.
- Usually The Law of Sines is easier to use than The Law of Cosines; so, if you have a choice, use the former.