One side of the rectangle is 3 dm 8 cm and its perimeter is 26 dm. find the other side of the rectangle.

1. For the convenience of calculations, we translate the known side of the rectangle into dm:
3 dm 8 cm = 3.8 dm.
2. From the formula for the perimeter of a rectangle P = 2a + 2b (P is the perimeter, a and b are the sides), we derive the formula for one side:
a = (P – 2b) / 2.
3. Find the unknown side of the rectangle:
a = (26 – 2 * 3.8) / 2 = (26 – 7.6) / 2 = 18.4 / 2 = 9.2 dm.
Answer: The unknown side of the rectangle is 9.2 dm.