When constructing a triangle given three side lengths, the first step involves:

The first step in constructing a triangle when given three side lengths typically involves drawing a line segment equal to the longest side. This is foundational because the longest side serves as a base from which the other two sides can accurately be positioned to form the triangle correctly.

In the process, after laying down the longest side, the next steps usually involve using a compass to measure arcs based on the lengths of the other two sides. This helps in locating the other two vertices of the triangle accurately.

While drawing arcs with a compass is essential in the construction process, it follows the initial step of establishing the longest side. Therefore, the construction begins with identifying and drawing the longest side first, making it critical to the triangle's formation.

Choosing the correct sequence is important in geometric constructions, as each step builds on the previous one to ensure precision in creating the figure.