Shortcut Trick
In any parallelogram, the diagonals bisect each other. Let the diagonals AC and BD of parallelogram ABCD intersect at point O.
Since O is the midpoint of diagonal AC, we have OA = OC.
Given that AE = FC, subtracting these equal lengths from OA and OC gives: OA − AE = OC − FC ⇒ OE = OF.
Thus, O is also the midpoint of segment EF, which means the diagonals BD and EF of quadrilateral BEDF bisect each other.
A quadrilateral whose diagonals bisect each other is always a parallelogram.
∴ The correct answer is parallelogram.
Alternate Method
Given:
⇒ ABCD is a parallelogram.
⇒ E and F are points on the diagonal AC such that AE = FC.
Properties Used:
⇒ In a parallelogram, opposite sides are equal and parallel.
⇒ SAS (Side-Angle-Side) Congruence Criterion: If two sides and the included angle of one triangle are equal to two sides and the included angle of another triangle, the triangles are congruent.
⇒ If both pairs of opposite sides of a quadrilateral are equal, then the quadrilateral is a parallelogram.
Calculations:
In triangles ΔADE and ΔCBF:
⇒ AD = BC (Opposite sides of parallelogram ABCD are equal)
⇒ ∠DAE = ∠BCF (Alternate interior angles, since AD || BC and diagonal AC is a transversal)
⇒ AE = FC (Given)
⇒ ΔADE ≅ ΔCBF (By SAS congruence criterion)
⇒ DE = BF (By CPCT) --- (Equation 1)
Similarly, in triangles ΔABE and ΔCDF:
⇒ AB = CD (Opposite sides of parallelogram ABCD are equal)
⇒ ∠BAE = ∠DCF (Alternate interior angles, since AB || CD and diagonal AC is a transversal)
⇒ AE = FC (Given)
⇒ ΔABE ≅ ΔCDF (By SAS congruence criterion)
⇒ BE = DF (By CPCT) --- (Equation 2)
In quadrilateral BEDF, from Equation 1 and Equation 2:
⇒ DE = BF and BE = DF
Since both pairs of opposite sides of quadrilateral BEDF are equal, BEDF is a parallelogram.
Why other options are incorrect:
⇒ Rectangle: A rectangle requires all interior angles to be 90°. Since the angles of the original parallelogram ABCD are not specified as 90°, the angles of BEDF are also not necessarily 90°.
⇒ Square: A square requires all sides to be equal and all angles to be 90°, which is not guaranteed here.
⇒ Trapezium, but not a parallelogram: A trapezium has only one pair of parallel sides, whereas BEDF has both pairs of opposite sides parallel and equal, making it a full parallelogram.
∴ The correct answer is parallelogram.
Additional Information
Properties of a Parallelogram
Opposite sides are equal and parallel, opposite angles are equal, consecutive angles are supplementary, and diagonals bisect each other.
Sufficient Conditions for a Quadrilateral to be a Parallelogram
A quadrilateral is a parallelogram if both pairs of opposite sides are equal, both pairs of opposite angles are equal, or its diagonals bisect each other.
SAS Congruence Criterion
If two sides and the included angle of one triangle are equal to two sides and the included angle of another triangle, then the two triangles are congruent.