Suppose A, B, and C are sets.
Prove that if A union B is a subset of C, then A is a subset of C and B is a subset of C.
My proof:
Suppose A, B, and C are sets such that A union B is a subset of C.
Then for all x, if x is in A union B, then x is in C.
Since x is in A union B, this means x...