Final answer:
The Law of Cosines is used to relate a triangle with two constant sides and one changing side, providing a relationship between the lengths of the sides and the cosine of the included angle. Therefore, the correct option is C.
Step-by-step explanation:
To relate a triangle with two constant sides and one side that changes in length, we can use the Law of Cosines, a variation of the Pythagorean theorem applicable to any triangle, not just right triangles. The Law of Cosines states that for any triangle with sides of length a, b, and c, and with C being the angle opposite side c, the following relationship holds: c² = a² + b² - 2abcos(C). When a triangle is not right-angled, using the Pythagorean theorem directly is inaccurate because the theorem only applies to right triangles. However, the Law of Cosines generalizes the concept by including an additional term to account for the included angle, thereby allowing us to find the length of the changing side based on the lengths of the other sides and the enclosed angle.