解：?$①$?因?yàn)?$[4，28]=x，$??$[7，28]=y，$?
所以?$4^{x}=28，$??$7^{y}=28。$?
則?$\frac {49^{y}}{64^{x}}=\frac {(7^2)^{y}}{(4^3)^{x}}=\frac {7^{2y}}{4^{3x}}=\frac {(7^{y})^2}{(4^{x})^3}=\frac {28^2}{28^3}=\frac {1}{28}。$?
?$ ②$?因?yàn)?$4^{xy}=(4^{x})^{y}=28^{y}，$??$4^{x + y}=4^{x}·4^{y}=28·4^{y}，$?
所以?$\frac {4^{xy}}{4^{x + y}}=\frac {28^{y}}{28·4^{y}}=\frac {1}{28}·\frac {28^{y}}{4^{y}}=\frac {1}{28}·(\frac {28}{4})^{y}$?
?$=\frac {1}{28}·7^{y}=\frac {1}{28}×28 = 1，$?
所以?$4^{xy}=4^{x + y}，$?
所以?$xy=x + y，$?
則?$t=\frac {x + y}{2xy}=\frac {1}{2}。$?
