Question 1136787
IDK anyone who would not assume the cross-section is circular when you say "pipe."
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Post it like this:
A rectangular pipe has a cross-section of 10 by 12 cms outside, and it's 1 cm thick.
etc.
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Makes it simple and understandable.
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Pipe ---> a round object.
You could have said "a rectangular wave guide" with dimensions.  tho they are copper, not iron.