The perimeter of the polygon ABCD is 29cm 8mm and AB = 6cm 9mm CD = 8cm 1mm AD = 5cm 9mm. Find the side length BC.

Let us determine what the total length of the sides AB and CD will equal, when from the condition of the task, it is known that the first is equal to 6 centimeters 9 millimeters, and the second is 5 centimeters 9 millimeters:

6 + 8 = 14;

9 + 1 = 10;

10 mm = 1 cm;

14 + 1 = 15;

15 cm.

Let’s determine what the total length of the three known sides will be equal to:

15 cm + 5 cm 9 mm = 20 cm 9 mm.

Let’s determine what the length of the fourth side will be if the perimeter of the figure is 29 cm 8 mm:

29 cm 8 mm – 20 cm 9 mm = 9 cm – 1 mm = 8 cm 9 mm.

Answer: 8 cm 9 mm.