The formula is Area = ½ times (Base times Height) or Area = (Base times Height) divided by 2. Finding the area of an isosceles or right triangle is easy as long as the length of the base and the height are known.
With a right triangle, the base is the bottom length. The height is the straight, non-angled line attached to the base where the 90-degree angle is formed. The other two angles are 45 degrees.
If the Base is 5 inches times a Height of 10 inches, you get 50 inches. Divide that result by two to find the Area, which is 25 inches.