We know what is a square in geometry and its properties. For finding area of square and perimeter of square we have formulas. When comes to **diagonal of a square**, does it has any formula? Yes it has the formula. Now we will see what is that formula for finding diagonal of a square.

First let us define a square. A quadrilateral which has all the sides equal and each angle is equal to 90 degrees is called square. A square contains two **diagonals** which are equal in length. Now in this concept we are going to learn how we can find diagonal of a square. So know the formula for length of **diagonal of a square** and also its proof that means how we got that formula.

**Diagonal of a square formula:**

Let PQRS be a square with ‘S ‘units as side.

PR is the diagonal in the above diagram.

Therefore, the diagonal PQ is given by \sqrt { 2 } S units. Square root 2 value just take approximate value i.e. 1.414.

**Proof for diagonal of a square formula:**

PQR is a right angled triangle.

We know that according to right angled triangle hypotenuse square is equal to sum of squares of remaining two sides.

Therefore, the diagonal of square is \sqrt { 2 } S units.

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