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cylinder1 - creates a cylinder that will form the inner wall of pipe 1with length 'L' that comes out of the page.
cylinder2 - creates a cylinder that will form the inner wall of pipe 2with length 'L/2'.
union1 - unions the two cylinders.
cylinder3 - creates a cylinder that will form the outer wall of pipe 1with length 'L'.
cylinder4 - creates a cylinder that will form the outer wall of pipe 2with length 'L/2'.
subtract1 - subtracts cylinder 1 from cylinder 3 the portion of cylinder2 and cylinder4 that are inside the cylinders creating a hollow pipe out of cylinder2 and cylinder4.
subtract2 - subtracts cylinder 2 3 from cylinder 4 1 creating a hollow pipe coming out of the page.
cylinder5 - creates a cylinder that will form the outer wall of pipe 3.
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