Final answer:
The Similarity Theorem is missing in the proof that uses similar triangles to establish the Pythagorean theorem, as it is vital for justifying the equating of side ratios in similar triangles.
Step-by-step explanation:
The justification missing in the proof of the Pythagorean theorem using similar triangles is the Similarity Theorem. When constructing a proof to establish that a² + b² = c² for a right triangle with hypotenuse c and legs a and b, the Similarity Theorem is crucial. It is used to show that the smaller triangles formed by drawing altitude CD are similar to the original right triangle ABC, which allows the properties of similar triangles to be applied. Without establishing the similarity of these triangles, the cross products of the sides cannot be properly equated, and the rest of the proof would not follow logically.