Derive Area of Triangle Formula

This example derives the area of a triangle formula by examining the three types of triangles: right triangles, acute triangles and obtuse triangles.

Case 1 (Right Triangle)

derive area of triangle right triangle case

The first case of a triangle is the right triangle case. Here we can observe that a rectangular region can be formed by drawing two congruent right triangles. From the area of a rectangle formula we know the area of this rectangle is equal to the base multiplied by the height so we can set the two equal to each other.

Solving for the area of the right triangle we are left with the area of triangle formula verifying the first case.

Case 2 (Acute Triangle)

derive area of triangle acute triangle case

The second case is an acute triangle where all three angles of the triangle are acute. In this case, we can divide the acute triangle into two right triangles as shown in the illustrations.

derive area of triangle acute triangle case

We can represent the area of the acute triangle as the sum of the two right triangles, shown below.

Then we can simplify, factoring out from both expressions and leaving us the expression below.

Then since we can substitute the variable back into the equation which leaves us with the same formoula, verifying the second case.

Case 3 (Obtuse Triangle)

derive area of triangle obtuse triangle case

The final case corresponds to the obtuse triangle shown in the illustration above. Here we can represent the area of the triangle as the area of the rectangle defined by the base and a height of minus the area of the two right triangles colored green. This is shown in the expression below.

Expanding the expression gives us the following expression.

Combining like expressions

Finally, we are left with the formula for area of the triangle.

This completes the derivation of the area of triangle formula for the three types of triangles: right, acute and obtuse.