How to solve a triangle given two sides and the angle between them.
This triangle has internal angles 'A', 'B' and 'C', and sides of length 'a', 'b' and 'c':
If three of these six measurements are known, then it may be possible to find the other three.
This is called 'solving' the triangle, and this topic will show you how to solve triangles for the unknown side and angles when any two sides and the included angle are given.
NOTE: The included angle is the angle between the two given sides.
These are the formulas used to solve triangles:
We will now use an example to show how these rules are applied to solve a triangle when two sides and the included angle are given.
Example: Solve this triangle for the unknown side, 'a', and internal angles 'B' and 'C':
Step 1: Begin by using the cosine rule to find the unknown side.
Step 2: Use the sine rule to find the smaller of the two unknown angles.
For the example triangle, side 'b' is smaller than side 'c', so angle B must be smaller than angle C. We choose to find angle B with the sine rule.
Step 3: Use the 'sum of internal angles' rule to find the third angle...
The triangle is now solved. This diagram shows all of the sides and angles:
Maths Helper Plus can solve a triangle given three side lengths. Full working steps and a labelled diagram are created. The steps below will show you how...
Step 2 Display the triangle solver options box
Double click the mouse in the border to the left of the calculations. ( This area is shaded pale blue in the diagram below.) The triangle solver options box will display its 'Lengths & Angles' tab...
Click the 'Clear' button to remove the previous triangle, then enter your given angle and two sides. You can enter these in three ways, but the answer will be correct as long as the given angle is between the given sides.
In the diagram above, we are entering sides 'b' and 'c', and the angle 'A' which is between 'b' and 'c'.
To enter a value, click on its edit box, then type the value.
NOTE: Angles are already in degrees. Do not follow with a degree symbol: °
Click the 'Apply' button at the bottom of the edit box. The calculated values will display on the options box.
Click the 'OK' button to close the options box. The calculations and triangle diagram will be displayed on your screen.
Step 3 Adjust the size of the diagram
If the triangle diagram is too big to display properly on your computer screen, briefly press the F10 key to reduce its size. To make the diagram bigger, hold down a Ctrl key while you press F10.