As we know, area of an equilateral Δ=√3/4×(side)²

∆=√3/4×(side)²

Let us assume

side of square = *a*

eq. ∆ drawn on side of sq. = *∆¹*

eq. ∆ drawn on diagonal = *∆²*

As we know, the diagonal of the square = √2×*a*

( by applying Pythagoras Theorem)

So, area of** ∆¹ **= √3/4×

*a²*area of *∆² **= *√3/4×(√2 ** a **)² =2[√3/4×

**]**

*a²*As we can clearly see

area of ** ∆² **=2×

*∆¹***Hence Proved.**

**Let us use an example as shown below:**

Here ABCD is a square, AEB is an equilateral triangle (** ∆¹)** described on the side of the square and DBF is an equilateral triangle (

**described on diagonal BD of the square.**

*∆²)*To Prove: Ar(ΔDBF) / Ar(ΔAEB) = 2 / 1

Proof: If two equilateral triangles are similar then all angles are = 60 degrees.

Therefore, by AAA similarity criterion , △*DBF* ~ △*AEB*

Ar(ΔDBF) / Ar(ΔAEB) = DB^{2} / AB^{2} --------------------(i)

We know that the ratio of the areas of two similar triangles is equal to

the square of the ratio of their corresponding sides i .e.

But, we have *DB *= √2*AB* {But diagonal of square is √2 times of its side} -----(ii).

Substitute equation (ii) in equation (i), we get

Ar(ΔDBF) / Ar(ΔAEB) = (√2*AB* )^{2} / AB^{2} = 2 AB^{2 }/ AB^{2 }= 2

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

STEP 1: Know about the equilateral triangle and it's properties

#### (https://brilliant.org/wiki/properties-of-equilateral-triangles/)

STEP 2: Use the given data in the question to make a figure.

STEP 3: Prove two equilateral triangles are similar [one Δ is on the side of the square and another one is on the diagonal]

(To prove this recall the criteria for similarity of triangles

1) AAA or AA 2) SSS 3) SAS. )

STEP 4: Conclude that the ratio of the areas of two similar triangles is equal to

the square of the ratio of their corresponding sides. If the equilateral triangles are similar.

\frac{Area(Δ on diagonal)}{Area( Δ on side)} = \frac{diagonal^2}{ side^2}

STEP 5: Use the Pythagoras theorem to calculate the diagonal of the square.

(The diagonal of square is √2 times of its side)