Proof
Lay the two parallelograms so the equal angles coincide; their union
forms a third parallelogram whose diagonal includes the original
common-angle vertex. By VI.1 the ratios of areas equal the ratios of
adjacent sides; equality of the original areas forces the
reciprocal-proportion relation. Converse runs the same way.
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Depends on (2)
Required by (dependents) (4)
- VI.15Proposition VI.15In equal triangles which have one angle equal to one angle the sides about the equal angles are reciprocally…
- VI.16Proposition VI.16If four straight lines be proportional, the rectangle contained by the extremes is equal to the rectangle contained by…
- XI.34Proposition XI.34In equal parallelepipedal solids the bases are reciprocally proportional to the heights; and those parallelepipedal…
- XII.9Proposition XII.9In equal pyramids which have triangular bases the bases are reciprocally proportional to the heights; and those…
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