Write down the three clues
2 pairs parallel;4 equal sides;equal diagonals All three must hold at once.
Use the first clue
2 pairs parallel⇒parallelogram Two pairs of parallel sides is the definition of a parallelogram.
Eliminate the kite
kite: 0 pairs parallel A kite that is not a rhombus has no parallel sides at all, so it fails the first clue.
Eliminate the one-pair trapezium
A trapezium with exactly one pair of parallel sides fails the first clue.
Eliminate the isosceles trapezium
An isosceles trapezium also has only one pair of parallel sides, so it fails too.
Use the second clue
4 equal sides⇒rhombus A parallelogram with all four sides equal is a rhombus.
Eliminate the rectangle that is not a square
A rectangle like (0, 0), (6, 0), (6, 3), (0, 3) has sides 6, 3, 6, 3, which are not all equal.
Eliminate the parallelogram with no right angles
AB2=25,BC2=13 The parallelogram (0, 0), (5, 0), (7, 3), (2, 3) has unequal adjacent sides, so its four sides are not all equal.
Take stock
it must be a rhombus The first two clues force the shape to be a rhombus. The third clue must now separate the square from the other rhombuses.
Test a rhombus that is not a square
(0,0),(5,0),(8,4),(3,4) This rhombus has diagonals with squared lengths 80 and 20.
Eliminate it
Its diagonals are NOT equal, so it fails the third clue. A rhombus that is not a square is ruled out.
Explain why equal diagonals force right angles
parallelogram+equal diagonals⇒rectangle In a parallelogram the diagonals are equal exactly when all four angles are right angles, which makes it a rectangle.
Combine the two names
rhombus+rectangle=square The shape is a rhombus (four equal sides) AND a rectangle (four right angles). A shape that is both is a square.
Check with a real square
(0,0),(4,0),(4,4),(0,4) Its diagonals are (4, 4) and (-4, 4), both with squared length 32, so they ARE equal. All four sides are 4. Every clue is satisfied.
State the answer
Square The only quadrilateral satisfying all three clues is the square.