Take nine numbers forming a square in a calendar and mark the four numbers at the corners.

Multiply the diagonal pairs and find the difference of these products.

3 × 19 = 57

17 × 5 = 85

85 – 57 = 28

Do this for other such squares.

Take nine numbers forming a square in a calendar and mark the four numbers at the corners.

Add the squares of diagonal pairs and find the difference of the sums.

3² + 19² = 370

17² + 5² = 314

370 – 314 = 56

Explain using algebra, why the difference is always 56. (it is convenient to take the number at the centre of the squares as x – see the section, Another trick of the lesson, Unchanging Relation of the Class 7 textbook)

Take nine numbers forming a square in a calendar and mark the four numbers at the corners.

Add the squares of diagonal pairs and find the difference of the sums.

3² + 19² = 370

17² + 5² = 314

370 – 314 = 56

Explain using algebra, why the difference is always 56. (it is convenient to take the number at the centre of the squares as x – see the section, Another trick of the lesson, Unchanging Relation of the Class 7 textbook)

Take nine numbers forming a square in a calendar and mark the four numbers at the corners.

Multiply the diagonal pairs and find the difference of these products.

3 × 19 = 57

17 × 5 = 85

85 – 57 = 28

Do this for other such squares.

Take nine numbers forming a square in a calendar and mark the four numbers at the corners.

Multiply the diagonal pairs and find the difference of these products.

3 × 19 = 57

17 × 5 = 85

85 – 57 = 28

Explain using algebra, why the difference is always 28 (It is convenient to take the number at the centre as x).