解：$(1) \frac {OC}{OD}=\frac {20}{24}=\frac 56，$$\frac {OA}{OB}=\frac 56$
∴$\frac {OC}{OD}=\frac {OA}{OB}，$$∠AOB=∠COD$
∴$△AOB∽△COD$
$(2) \frac {ED}{DF}=\frac {30}{48}=\frac 58，$$\frac {AB}{AC}=\frac {45}{72}=\frac 58$
∴$\frac {ED}{DF}=\frac {AB}{AC}$
又$∠A=∠D$
∴$△EDF∽△BAC$
