Prove that if
g: A -> B is 1-1
f: B -> C is 1-1
then fog: A -> C is 1-1
pf.
Need to show that if f(g(a)) = f(g(c)) then a = c for fog to be 1-1
Because g is 1-1, g(a) = g(b) and a = b
Because f is 1-1, f(b) = f(c) and b = c therefore a = c
Therefore since a = c, fog is a 1-1 function.