Often, a text on differential equations will mention "linear combination" of functions. What exactly does that phrase mean? I've been fuzzy on it all semester. The day before the final is a good time to ask, ehe?

For example, there's a question that asks,

Prove that any two-dimensional vector can be written as a linear combination ofi+jandi-j.

Just as an example, but by all means please use examples that pop into your own mind.

Thanks.