4) c2 = h2 + b2 -2bx + x2

The terms b² − 2bx + x² come from expanding (b − x)² using (u − v)² = u² − 2uv + v². Thus c² = h² + (b − x)² becomes c² = h² + b² − 2bx + x². The correct choice is “Expand (b – x)².”

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