Shortcut Trick
A square is a regular quadrilateral with highly symmetric properties:
- All angles equal: Each interior angle is exactly 90° (Statement a is true).
- Diagonals equal: Both diagonals are equal in length (Statement c is true).
- All sides equal: All four sides have the same length (Statement d is true).
- Sides parallel: Only opposite sides are parallel. Adjacent sides are perpendicular, so 'all sides parallel' is false (Statement b is false).
Thus, statements a, c, and d are true.
∴ The correct answer is a, c, d.
Alternate Method
Let ABCD be a square with side length a.
Let us evaluate each statement mathematically:
⇒ Statement a: All angles equal
In a square, the sum of interior angles is 360°. Since it is a regular quadrilateral, each interior angle is:
⇒ ∠A = ∠B = ∠C = ∠D = 360° ÷ 4 = 90°.
Since all four angles are equal to 90°, this statement is true.
⇒ Statement b: All sides parallel
By definition, parallel lines never intersect. In a square:
⇒ AB is parallel to CD (AB || CD) and AD is parallel to BC (AD || BC).
However, adjacent sides intersect at a 90° angle (e.g., AB is perpendicular to BC). Therefore, all sides are not parallel to one another. This statement is false.
⇒ Statement c: Diagonals equal
Using the Pythagorean theorem in right-angled triangle ABC:
⇒ Diagonal AC = √(AB2 + BC2) = √(a2 + a2) = a√2
Similarly, in right-angled triangle ABD:
⇒ Diagonal BD = √(AD2 + AB2) = √(a2 + a2) = a√2
Since AC = BD = a√2, the diagonals are equal in length. This statement is true.
⇒ Statement d: All sides equal
A square is defined as a regular quadrilateral, which means it is equilateral:
⇒ AB = BC = CD = DA = a.
Since all four sides are equal in length, this statement is true.
⇒ Thus, statements a, c, and d are correct.
∴ The correct answer is a, c, d.
Additional Information
Diagonals of a Square
The diagonals of a square are equal, bisect each other at right angles (90°), and bisect the interior angles into 45° angles.
Area and Perimeter Formulas
For a square of side length a and diagonal d, the Area = a2 = d2 ÷ 2, and the Perimeter = 4 × a.
Symmetry of a Square
A square has 4 lines of symmetry (2 diagonal lines and 2 lines passing through the midpoints of opposite sides) and rotational symmetry of order 4.