Tokyo stocks opened mixed Friday as buying tracking overnight gains on Wall Street was offset by selling after the securing of dividend rights before the end of the fiscal first half. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average rose 288.10 points, or 0.74 percent, from Thursday to 39,213.73. The broader Topix index was down 1.78 points, or 0.07 percent, at 2,719.34. On the top-tier Prime Market, gainers were led by machinery, electric appliance and chemical shares. The main decliners were mining, oil and coal product, and pulp and paper shares. At 9 a.m., the U.S. do…