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III. Two regular polyedrons of the same name are two similar solids, and their homologous dimensions are proportional; hence the radii of the inscribed or of the circumscribed spheres are to each other as the sides of the polyedrons.

iv. If a regular polyedron is inscribed in a sphere, the planes drawn from the centre, along the different edges, will divide the surface of the sphere into as many spherical polygons, as the polyedron has faces all equal and similar among themselves.

Improved Demonstration of the Theorem for the
Solidity of the Triangular Pyramid:



568. Two triangular pyramids, having equivalent bases and equal altitudes, are equivalent, or equal in solidity.

Let S-ABC, s-abc (fig. 294) be two triangular pyramids of Fig. 294. which the two bases ABC, a b c, supposed to be situated in the same plane, are equivalent, the altitude TA being the same in both. If they are not equivalent, let s-a b c, be the smaller; and suppose A a to be the altitude of a prism, which having ABC for its base, is equal to their difference.

Divide the altitude AT into equal parts Ax, xy, yz, &c., each less than A a, and let k be one of these parts; through the points of division suppose planes parallel to the plane of the bases; the corresponding sections formed by these planes in the two pyramids will be respectively equivalent by art. 409, namely, DEF, to def, GHI, to g h i, &c.

This being granted, upon the triangles ABC, DEF, GHI, &c., taken as bases, construct exterior prisms having for edges the parts AD, DG, GK, &c., of the side SA; in like manner, on the bases def, ghi, k l m, &c., in the second pyramid, construct interior prisms having for edges the corresponding parts of s a. It is plain that the sum of all the exterior prisms of the pyramid S-ABC will be greater than this pyramid; and also that the sum of all the interior prisms of the little pyramid s-a b c will be less than this. Hence the difference between the sum of all the exterior prisms and the sum of all the interior ones, must be greater than the difference between the two pyramids themselves.

Now, beginning with the bases ABC, abc, the second exterior prism DEFG is equivalent to the first interior prism defa, because they have the same altitude k, and their bases DEF, def, are equivalent; for like reasons, the third exterior prism GHIK and the second interior prism ghid are equivalent; the fourth exterior and the third interior; and so on, to the last in each series. Hence all the exterior prisms of the pyramids S-ABC, excepting the first prism DABC, have equivalent corresponding ones in the interior prisms of the pyramid s-abc; hence the prism DABC is the difference between the sum of all the exterior prisms of the pyramid S-ABC; and the sum of all the interior prisms of the pyramid s-ab c. But the difference be tween these two sets of prisms has already been proved to be greater than that of the two pyramids; which latter difference we supposed to be equal to the prism a ADC; hence the prism DABC must be greater than the prism a ABC. But in reality it is less; for they have the same base ABC, and the altitude Ax of the first is less than A a the altitude of the second. Consequently the supposed inequality between the two pyramids cannot exist; therefore the two pyramids S-ABC, s-a bc, having equal altitudes and equivalent bases, are themselves equivalent.

Fig. 216.


569. Every triangular pyramid is a third part of the triangular prism having the same base and same altitude.

Demonstration. Let F-ABC (fig. 216) be a triangular pyramid, ABCDEF a triangular prism of the same base and the same altitude; the pyramid will be equal to a third of the prism.

Cut off the pyramid F-ABC from the prism, by a section made along the plane FAC; there will remain the solid FACDE, which may be considered as a quadrangular pyramid, whose vertex is F, and whose base is the parallelogram ACDE. Draw the diagonal CE; and extend the plane FCE, which will cut the quadrangular pyramid into two triangular ones F-ACE F-CDE. These two triangular pyramids have for their common altitude the perpendicular let fall from F on the plane ACDE; they have equal bases, the triangles ACE, CDE, being halves of the same parallelogram; hence (568) the two pyramids F-ACE, F-CDE, are equivalent. But the pyramid F-CDE

and the pyramid F-ABC have equal bases ABC, DEF; they have also the same altitude, namely, the distance of the parallel planes ABC, DEF; hence the two pyramids are equivalent. Now the pyramid F-CDE has already been proved equivalent to F-ACE; consequently the three pyramids F-ABC, F-CDE, F-ACE, which compose the prism ABD are all equivalent. Therefore the pyramid F-ABC is the third part of the prism ABD, which has the same base and the same altitude.

570. Corollary. The solidity of a triangular pyramid is equal to a third part of the product of its base by its altitude.

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