The common formula to find the area of an isosceles triangle is area equals 1/2 * base * height of triangle. The height is the line bisecting the two equal sides of the triangle and intersects the midpoint of the base.
In general, the formula for a triangle's area is area equals 1/2 * base * height. The side of the triangle that has a different length than the other two is the base; the height of the triangle is the length from the vertex of the two equivalent sides to the midpoint of the base. If "a" is the length of the base and "b" is the length of the two equivalent sides, then the Pythagorean theorem shows that another formula for area of an isosceles triangle is area = 1/2 * a^2 * sqrt((b^2 / a^2) - 1/4).