Part A:
By Angle-Angle Theorem,
Triangle BCA is similar with triangle BDC (∠B ≅ ∠B, and ∠C ≅ ∠D)
Triangle CDA is similar with triangle BCA (∠CBA ≅ ∠DCA, and ∠BCA ≅ ∠CDA)
By transitive property
ΔBCA ~ ΔCDA ~ BDC
Part B:
Part C:
Cross multiplying the proportions we have