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PROPOSITION IV. THEOREM. 529. An oblique prism is equivalent to a right prisi whose bases are equal to right sections of the oblique prism, and whose altitude is equal to a lateral edge of the oblique prism.
Let A D' be an oblique prism, and FI a right section.
Complete the right prism Fl', making its edges equal to those of the oblique prism.
We are to prove oblique prism A D' right prism FI.
§ 492 (two trihedrals are equal when three face É of the one are respectively equal to three face of the other, and are similarly placed). Now face A D= face A' D',
$ 505 (being the two bases of the oblique prism A D'); face A J=face A' J',
Cons. and face A G = face A' Gʻ.
Cons. .. prism A I = prism A' I',
$ 527 (two truncated prisms are equal when the three faces including a trihedral
of the one are respectively equal to the three faces including a trihedral
Q. E. D. .
PROPOSITION V. THEOREM. 530. Any two opposite faces of a parallelopiped are equal and parallel.
Let AG be a parallelopiped.
§ 517 A B and D C are equal and il lines. § 125 Also, since A H is a o,
$ 505 A E and D H are equal and || lines. § 125 .. L E AB= _ HDC,
§ 462 (two ts not in the same plane having their sides II and lying in the same
direction are equal).
§ 140 Moreover, face A F is || to DG
§ 463 (if two & not in the same plane have their sides II and lying in the same
direction their planes are parallel). In like manner we may prove A H and B G equal and parallel.
Q. E. D. 531. SCHOLIUM. Any two opposite faces of a parallelopiped may be taken for bases, since they are equal and parallel parallelograms.
PROPOSITION VI. Theorem. 532. The plane passed through two diagonally opposite edges of a parallelopiped divides the parallelopiped into two equivalent triangular prisms.
Let the plane A EGC pass through the opposite edges
A E and C G of the parallelopiped A G.
We are to prove that the parallelopiped AG is divided into two equivalent triangular prisms, A B C-F, and A D C-H.
Let IJKL be a right section of the parallelopiped made by a plane I to the edge A E.
The intersection I K of this plane with the plane A EGC is the diagonal of the OIJKL.
AIKJ=A IKL. . § 133 But prism A B C-F is equivalent to a right prism whose base is I J K and whose altitude is A E,
$ 529 (any oblique prism is to a right prison whose. bascs are equal to right sec
tions of the oblique prism, and whose altitude is equal to a lateral edge of the oblique prism).
The prism A D C-H is equivalent to a right prism whose base is I L K, and whose altitude is A E.
§ 529 Now the two right prisms are equal, (two right prisms having equal bases and altitudes are equal). .. A B C-F A DC-H.
Q. E. D.
PROPOSITION VII. THEOREM. 533. Two rectangular parallelopipeds having equal bases are to each other as their altitudes.
Let A B and A'B' be the altitudes of the two rectangular parallelopipeds, P, and P', having equal bases.
P A B
Find a common measure m, of A B and A' B'. Suppose m to be contained in A B 5 times, and in A' B' 3 times.
Then we have AB=3
At the several points of division on A B and A' B' pass planes I to these lines.
The parallelopiped P will be divided into 5,
Then we have
CASE II. — When A B and A' B' are incommensurable.
Let A B be divided into any number of equal parts,
and let one of these parts be applied to A' B' as many times as A' B' will contain it.
Since A B and A' B' are incommensurable, a certain number of these parts will extend from A to a point D, leaving a remainder D B' less than one of these parts.
Through D pass a plane I to A' B', and denote the parallelopiped whose base is the same as that of P', and whose altitude is À' D by. Q. Now, since A B and A' D are commensurable, Q:P = A' D : A B.
(Case I.) Suppose the number of parts into which A B is divided to be continually increased, the length of each part will become less and less, and the point D will approach nearer and nearer to B'.
The limit of Q will be P',
.. the limit of Q:P will be P': P, and the limit of A' D : A B will be A' B' : A B, Moreover the corresponding values of the two variables Q:P and A' D:AB are always equal, however near these variables approach their limits. .. their limits P: P= A' B' : A B. § 199
Q. E. D. 534. SCHOLIUM. The three edges of a rectangular parallelopiped which meet at a common vertex are its dimensions. Hence two rectangular parallelopipeds which have two dimensions in common are to each other as their third dimensions.