The large side of the parallelogram is 5 cm. Find the second side of the parallelogram if the perimeter is 18 cm.

Given:
parallelogram AВСK,
AK = 5 centimeters,
R ABСK = 18 centimeters.
Find the length of the side of the parallelogram AB -?
Decision:
Consider the parallelogram ABСK. In it, the opposite sides are equal to each other, that is, AB = СK and AK = BC = 5 centimeters.
The perimeter of a parallelogram is the sum of the lengths of all its sides. We get:
R ABCК = AB + СK + AK + BC;
18 = AB + СK + 5 + 5;
18 = AB + СK + 10;
AB + СK = 18 – 10;
AB + СK = 8;
AB = СK = 8: 2;
AB = СK = 4 centimeters.
Answer: 4 centimeters