Tokyo stocks opened mixed Thursday as buying following Wall Street gains overnight was offset by selling of tech shares amid caution before an earnings report from a major Taiwanese chipmaker later in the day. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 3.10 points, or 0.01 percent, from Wednesday to 39,177.20. The broader Topix index was up 8.71 points, or 0.32 percent, at 2,699.37. On the top-tier Prime Market, gainers were led by electric power and gas, and air transportation issues, while top decliners were precision instrument and nonferrous metal issues. A…