# Chapter 2 - Reasoning and Proof - 2-5 Reasoning in Algebra and Geometry - Practice and Problem-Solving Exercises: 25

The error is in step 5, where the Division Property of Equality was used to divide by $(b-a)$. Since it was given that $b=a$, we can calculate that $b-a=0$. Division by 0 is undefined, so this step of the proof is invalid.

#### Work Step by Step

Since division by 0 is undefined, you cannot include a step in the proof that would involve division by 0.

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