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There are some properties which are always true for any parallelogram.
Let's see those properties with explanation.
In a parallelogram, the following properties are true:
- the opposite sides are equal in length.
- the opposite angles are equal in measure.
- the adjacent angles are supplementary.
- the diagonals bisect each other.
Proof:
Let's prove (1) and (2).
Let ABCD be a parallelogram.

Draw a diagonal BD and denote the interior angles as ∠1, ∠2, ∠3 and ∠4.
Since ABCD is a parallelogram, AD is parallel to BC and AB is parallel to CD.
Consider the parallel lines AD and BC and take the diagonal BD as transversal.
Here ∠2 = ∠4 by alternate interior angle property [Alternate interior angles are equal in measure].
Now consider parallel lines AB and CD and take the diagonal BD as transversal.
Here ∠1 = ∠3 by alternate interior angle property [Alternate interior angles are equal in measure].
So we have ∠1 + ∠2 = ∠3+∠4. That is, ∠B = ∠D.
Consider the triangles DAB and BCD with the common side (BD = BD).
Now we have ∠2 = ∠4, ∠1 = ∠3 and the common side (BD = BD).
By ASA criterion of congruence, DAB BCD.
That is, they are congruent triangles.
Therefore, AD = BC, AC = CD and ∠A= ∠C.
Hence, in parallelogram ABCD, we have AD = BC, AC = CD, ∠A = ∠C and ∠B = ∠D.
This proves (1) and (2).
From (2), it is obvious that ∠A = ∠C and ∠B = ∠D.
Let's prove (3).
Let the measure of angles are∠A = ∠C = x and ∠B = ∠D = y.
We have the property that sum of all interior angles of a quadrilateral is 360°.
That is, ∠A + ∠B +∠C + ∠D = 360°.
Substituting the taken values in the above equation.
x + y + x + y = 360°
2x +2y = 360°
2(x + y) = 360°
x + y = 180°.
That is, we can write this as ∠A + ∠B = 180°, ∠B + ∠C = 180°, ∠C + ∠D = 180° and ∠A + ∠D = 180°.
It proves the property (3).
Let's prove (4).
Consider a parallelogram ABCD. Draw its diagonals AC and BD. Let the intersection point of the diagonals be O.
In triangle AOB and COD, we have:
AB = CD as opposite sides are equal in parallelogram.
∠AOB = ∠COD [Because vertically opposite angles are equal].
Here AB is parallel to CD, so ∠OAB = ∠DCO.
By AAS criterion of congruence, OABOCD.
This implies, OA = OC and OB = OD.
As they are equal, diagonals of a parallelogram bisect each other.