Test and analysis: △ Abd △ ECD can be proved by ∠ b = ∠ c, ∠ ADB = ∠ EDC, and then the result can be obtained according to the properties of similar triangles.

∠B =∠C，∠ADB =∠EDC

∴△ABD∽△ECD

∴

∴ AB = =100 (m)

Answer: The river width AB is 100 meters.

Comments: The key to solving the problem is to master the proportion of the corresponding side of similar triangles, and pay attention to writing the corresponding letters in the corresponding position.