Show that whenever value distributions are independent, there exist reserves for which Vickrey auction obtains 2-approximation to the optimal auction single-item

3.3 The Vickrey auction with rescrve prices R=(r1,,rn), sells an item to the bidder i with bid biri that has the highest bid, if such a bidder exists. The payment that this winning bidder makes is the maximum of their reservation price ri and the bid of the highest other bidder j that bids at least their reservation price, rj