Measure the perpendicular distance from the obtuse angle's corner to the side opposite it. For this example, allow the distance to measure 2 inches.
Multiply the length of the perpendicular distance by the length of the side to which it is perpendicular. In this example, allow the side to be 6 inches. Multiplying 2 inches by 6 inches results in 12 square inches.
Divide the product from Step 2 in half. Concluding this example, dividing 12 square inches by 2 results in 6 square inches. The area of the example triangle is 6 square inches.