The question in the image asks to complete the statement:

"Четырёхугольник является ___, если его противоположные ___ попарно равны."
Translation:
"A quadrilateral is a ___ if its opposite ___ are pairwise equal."

Solution:

1. The statement refers to a parallelogram because:
   • One of the key properties (свойство) of a parallelogram is that opposite sides (противоположные стороны) are equal in length.

Completed Sentence:

Четырёхугольник является параллелограммом, если его противоположные стороны попарно равны.

Final Answer:

• First blank: параллелограммом
• Second blank: стороны

Would you like an explanation on properties of parallelograms or how to identify their signs and characteristics?

5 Related Questions:

1. What are the properties of a parallelogram?
2. How do you prove that a quadrilateral is a parallelogram?
3. What are the signs (признаки) of a parallelogram?
4. How is a parallelogram different from a trapezoid?
5. How do the diagonals behave in a parallelogram?

Tip: In a parallelogram, not only are opposite sides equal, but opposite angles are also equal!