The first six books of the elements of Euclid : with a commentary and geometrical exercises ; to which are annexed a treatise on solid geometry, and short essays on ancient geometrical analysis, and the theory of transversals (1861)
Lardner, Dionysius, 1793-1859 · London : Print. for Taylor and Walton · leaf 256
. (42) The parts of parallel lines intercepted between parallel planes are equal. Let E F and G H be the parallel planes, and A B and C Dthe parallel lines. Draw AC and BD. Since A C and B D are in parallel planes theycannot meet, and since also they are in the sameplane (that of the parallels)… … Proposition XXI. (44) If two angles have their sides respectively paralleland lying in the same direction, they will beequal, and their planes will be parallel. If they be in the same plane, the proposition has been provedin the Elements (117). If they be not in the same plane, let the angles be B…
Geometry
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