ORCCAOpen Resources for Community College Algebra

SectionA.3Triangle

In Figure A.3.1, there are $3$ triangles. From left to right, there is an acute triangle, a right triangle and an obtuse triangle. Notice how to draw a height for an obtuse triangle.

The perimeter of a triangle is easy to calculate—simply add up the length of its $3$ sides.

To derive a triangle's area formula, let's look at the following figure:

If we reflect (or flip) a triangle by one of its sides, we will get a parallelogram or rectangle whose area is twice the triangle's area. In other words, a triangle's area is half the area of a parallelogram with the same base and height. Since a parallelogram's area formula is "base times height", we have:

where $b$ stands for a triangle's base and $h$ for its height.