If one side of a triangle is 18 cm, and the other two are 9 cm, then the perimeter of this triangle is 36 cm. What is the error?

Let’s denote the sides of the triangle through:

a = 9

b = 9

c = 18

Any side of a triangle is less than the sum of the other two sides and more
their differences.

The expression c> a + b is not valid.

Therefore, the figure indicated in the conditions of the problem is not a triangle.

Answer: an error in the formulation of the problem, the figure is not a triangle.