In the diagram below, ABCD is a rectangle.

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  BC = 10 ft

Step-by-step explanation:

You may recognize the ratio of the given leg and hypotenuse of triangle BCD is 24 : 26 = 12 : 13. You may recall that the Pythagorean triple (5, 12, 13) is commonly seen in math problems. Here, that means side BC is 2×5 = 10 feet.

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If you don't recall these helps, you can use the Pythagorean theorem to find BC.

  BC² + DC² = DB²

  BC² = DB² -DC² = (26 ft)² -(24 ft)² = 676 ft² -576 ft² = 100 ft²

  BC = √(100 ft²) = 10 ft

it would open into a square depending on the type of envelope i’m not sure about the second part