Answer:
When the opposite sides of the trapezoid are parallels each other and the sumatory of the inner angles is equal to 180°.
Step-by-step explanation:
The definition of a trapezoid is: a quadrilateral figure that only have two parallels sides and unequal each other. There are diferent types of trapezoids and can be classifiyed according to there inners angles. The rectangulum trapezium has two consecutive angles of 90°. The isosceles trapezium has two and two aquals angles, and the scalen trapezium has the four inner angles unequal each other.
A parallelogram is a quadrilateral figure too but a parallelogram has all his sides parallels each others. Examples of parallelograms are the recntagle, the rhomb and the romboid.
In conclusion we can say that a trapezoid is a parallelogram when the opposites sides of the tifgure are totally parallels and the sumatory of the iner angles is equal to 180°.
