Question

In order to use the Pythagorean theorem to find the magnitude of a resultant vector, which must be true regarding the two initial vectors?

Answer 1

We know that the Pythagorean theorem applies only to right triangles, therefore the original vectors must be parallel to the legs of a right triangle.

In other words, they must be orthogonal (i.e. perpendicular to each other) in order that the Pythagorean theorem applies.