Final answer:
The perimeter of a rectangle varies jointly with the sum of its side lengths, with a constant of proportionality of 2.
Step-by-step explanation:
The subject of this question is Mathematics. The question is asking about the variation of the perimeter of a rectangle with the sum of its side lengths. In this case, the perimeter is jointly proportional to the sum of the lengths of its sides, and the constant of proportionality is 2.