Heron gave this formula to find area of any triangle and that is = Square root under {s(s-a)(a-b)(s-c)} sq. units where s=(a+b+c)/2 and a, b and c are the sides of the triangle.
From this, two formulas are derived:-
1st for equilateral triangle and that is =(root under 3)/4*a^2 sq. units where a is the side of equilateral triangle.
And 2nd for isosceles triangle =1/2*b/4*Root under 4a^2-b^2 sq. units