• ΔACB∼ΔADC by AA (Angle Angle Postulate)  each Δ has a right angle and share ∠A.
• ΔACB∼ΔCDB by AA (Angle Angle Postulate)  each Δ has a right angle and share ∠B.
• We can establish that ∠B ∠ACD because they are each complementary to ∠DCB.
ΔADC∼ΔCDB by AA  each Δ has a right angle and ∠B ∠ACD.
Since these triangles are similar, we can establish a series of proportions relating their corresponding sides. Two valuable theorems are formed using 3 of these proportions:

Remember the "look" of the given diagram for this theorem. If you "forget" the rules stated in the following theorems, you can simply recall this original diagram and set up the corresponding sides of the three similar triangles.
