Show that the following are equivalent:

1: The random variables X1 and X2 are said to be independent iff for any linear Borel sets {Bj, 1<=j<=n} we have P{(X1 in B1) and (X2 in B2)}=P(X1 in B1)P(X2 in B2)

2: For a,b in R
P{(X1 <= a) and (X2 <=b)} = P(X1<=a)P(X2<=b)


Any ideas??