解：?$(1)①tan A=\frac {BC}{AC}=\frac 34，$??$tan B=\frac {AC}{BC}=\frac 43$?
②在?$Rt△ABC$?中，∵?$AB=13，$??$AC=5$?
∴?$BC=\sqrt {AB^2-AC^2}=12$?
∴?$tan A=\frac {BC}{AC}=\frac {12}{5}，$??$tan B=\frac {AC}{BC}=\frac {5}{12}$?
③在?$Rt△ABC$?中，∵?$AB=25，$??$AC=24$?
∴?$BC=\sqrt {AB^2-AC^2}=7$?
∴?$tan A=\frac {BC}{AC}=\frac {7}{24}，$??$tan B=\frac {AC}{BC}=\frac {24}{7}$?
?$(2)$?當(dāng)?$∠A+∠B=90°$?是，?$tan A · tan B=1，$?即互余兩角的正切互為倒數(shù)
