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What's the size of K in this figure?

enter image description here

A and a are parallel B and b are parallel

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Here's the diagram with some new lines drawn: enter image description here

Because the line segments of length $A$ and $a$, and the line segments of length $B$ and $b$, respectively, are parallel, the triangles are right triangles. Therefore, by the Pythagorean theorem, we have that $K^2=(A-a)^2+d^2$ and $K^2=(B-b)^2+d^2$, so $$K=\sqrt{(A-a)^2+d^2}=\sqrt{(B-b)^2+d^2}.$$

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