

1) ∠ADC = ∠BAC  1) Given 
2) ∠C = ∠C  2) Reflexive (common) 
3) ΔABC ~ ΔDAC  3) AA criteria (postulate) 
4) AB/DA = CB/CA = CA/CD  4) If two triangles are similar then their sides are in proportion. 
5) CB/CA = CA /CD  5) Last two ratios 
6) CA^{2} = CB x CD  6) Cross multiplication . 