How to solve a triangle given two sides and a non-included angle.
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 a non-included angle are given.
NOTE: The non-included angles are the angles that do not lie 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 a non-included angle are given.
Example: A triangle has sides a=5 and b=7, and a non-included angle A=30º. Solve for the unknown side and the two unknown angles.
Often this type of triangle problem has two solutions. In this case, there are two possible triangles that can be constructed with this information (See case 1 and case 2 below.) The reason for the two answers will be explained later.
Step 1: Begin by using the sine rule to find the unknown angle opposite one of the given sides.
Step 2: Find the remaining unknown angle.
Step 3: Use the sine rule to find the remaining unknown side.
The triangle is now solved. This diagram shows both case 1 and case 2 solutions on the same diagram:
The large blue triangle is the case 1 solution. The sides and angles marked on the diagram are all for the case 1 solution. The case 2 solution is the smaller shaded triangle with one red side. The red side is side 'a', which can have 2 possible positions. This is how the two triangles are created. If side 'a' is just long enough to reach the base line, then there is only one solution, and angle B is a right angle. If side 'a' is too short to reach the base line, then there are no solutions possible.
Maths Helper Plus can solve a triangle given two sides and a non-included angle. 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 click on the 'a' edit box. Type the new length for side 'a' of your triangle. Repeat for side 'b' and the non-included angle 'C'.
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.