Part a: The triangles ∆ABC and ∆EDC are similar by AA similarity rule.
Part b: The width of AB is 67.5 feet
Explanation:
Part a: We need to prove that the two triangles ABC and EDC are similar.
To prove the triangles are similar, then their angles must be similar.
Thus, we have,
∠DCE and ∠BCA are similar (vertical angles)
∠CDE and ∠CBA are similar (right angles)
∠B and ∠A are similar
Hence, the triangles ∆ABC and ∆EDC are similar by AA similarity rule.
Part b: We need to determine the width of AB
Since, the triangles are similar, then their corresponding lengths are proportional.
Thus, we have,

where
,
and 
Substituting these values, we get,

Multiplying both sides by 90, we get,


Dividing, we have,

Thus, the width of the river AB = 67.5 feet