Sept 1: Understand the question and construct a diagram through the given hints



                  Here ABCD is a square,

                Equilateral \triangle DBF on the diagonal of a square

                Equilateral \triangle AEB on the side of the square.

Step 2: Show that the two equilateral triangles are similar

            NOTE: In every equilateral triangle's angle = 60 degrees.

            Therefore, by AAA similarity criterion , 

            Equilateral triangles are similar   \triangle DBF ~  \triangle AEB

Step  3:  Find the ratio of the areas of the smilar triangle.

              THEOREM:  theorem: The ratio of the areas of two similar triangles is

              equal to the ratio of the squares of their corresponding sides.)

              =   \frac{area \triangle DBF}{area \triangle AEB} = (\frac{DB}{AB})^2 ........................(1)

Step 3: Recall the relation between the side and diagonal of the square

            NOTE: Diagonal of square is  \sqrt{2} times of its side

            EXAMPLE: From the figure  DB = \sqrt{2} AB 

Step 4: Plugging the diagonal (DB) value in the equation (1)

          EXAMPLE: \frac{area \triangle DBF}{area \triangle AEB} = (\frac{DB}{AB})^2

= \frac{area \triangle DBF}{area \triangle AEB} = (\frac{\sqrt{2}AB}{AB})^2

= \frac{area \triangle DBF}{area \triangle AEB} = \frac{2}{1}

= Area \triangle DBF = 2*area \triangle AEB

Hence proved

∴ Area of equilateral triangle described on one side of square is equal to half the area of the equilateral triangle described on one of its diagonals.