Final answer:
The statement is true because z, being a member of Span(x, y), implies that it can be expressed as a linear combination of x and y resulting in a linearly dependent set.
Step-by-step explanation:
The statement is true that if x and y are linearly independent, and if z is in Span(x, y), then the set {x, y, z} is linearly dependent. This is because z can be expressed as a linear combination of x and y. Therefore, there exists a non-trivial combination of x, y, and z that equals the zero vector, which is the definition of linear dependence.