An obtuse triangle is any triangle that contains an obtuse angle -- an angle that is greater than 90 degrees. The formula for finding the area of an obtuse triangle is the same as for other triangles, area = 1/2 x (base x height). However, because an obtuse triangle's height does not match the height of any of its sides, the method of finding the height of the figure is different.
Finding the Height
To calculate the area of an obtuse triangle, first find the height of the figure. Extend the base of the figure with a dotted line so that it is at least as far out from the rest of the figure as the top vertex. From the end of this line, draw a vertical dotted line all the way up to the top vertex of the figure to create a 90-degree angle. Measure this vertical line to get the triangle's height.
Finding the Area
Once you have the height of your triangle, find the length of the base. For an obtuse triangle, any side of the figure can be considered the base, so measure one of the sides and insert it into the formula area = 1/2 x (base x height). For example, if the base is 3 and the height is 6, your calculation would be 1/2 times 3 times 6 equals 9. Written out, it would look like this: 1/2 (3 x 6) = 9. Therefore, the area of the triangle is 9.