#### Answer

a. They are parallel.
b. They are perpendicular.
c. The second vector has a magnitude of 0.

#### Work Step by Step

a. The two vectors are parallel since the length of the sum is the sum of the lengths.
b. The two vectors must be at right angles to each other because the magnitude of the sum is found using the Pythagorean theorem.
c. Vector 2 must be the zero vector if neither adding it nor subtracting it from the first vector alters its value.