DALLAS — The Lakers were halfway to their third victory in four games on their last trip of significance this season. Andrew Bynum had 12 points on 6-for-8 shooting plus seven rebounds to lead the Lakers in the first half tonight against the Dallas Mavericks. Pau Gasol and Kobe Bryant each scored 10. Bryant made only 4 of 11 shots. Shawn Marion scored 15 for Dallas, which led the Lakers by 1 1/2 games in the Western Conference standings and had won 20 of 23 games going into tonight. Dallas shot only 17 of 44 (38.6 percent).