Problem
The two straight lines $EF$ and $GH$, which are perpendicular to the two parallel lines $AB$ and $CD$, respectively, are parallel to each other.
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Solution
$$GH⊥CD,$$
$$CD∥AB,$$
$$∴ \ GH⊥AB,$$
$$EF⊥AB,$$
$$∴ \ GH∥EF.$$
$$CD∥AB,$$
$$∴ \ GH⊥AB,$$
$$EF⊥AB,$$
$$∴ \ GH∥EF.$$
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Reference
Teiichiro Sasabe (1976) The Encyclopedia of Geometry (2nd edition), Seikyo-Shinsha, p.7