1 contained by three plane angles. Which was to be done. PROP. XXVII. THEOR. To describe from a given straight line a solid parallelo- See N. piped similar and similarly situated to one given. Let AB be the given straight line, and CD the given solid parallelopiped. It is required from AB to describe a solid parallelopiped similar and similarly situated to CD. At the point A of the given straight line AB make* 26. 11. a solid angle equal to the solid angle at C, and let BAK, KAH, HAB, be the three plane angles which contain it, so that BAK be equal to the angle ECG, and KAH to GCF, and HAB to FCE: and as EC to CG, so make* BA to AK; and as GC to CF, so make* KA to AH; wherefore, ex æquali *, as EC to 12. 6. CF, so is BA to AH: complete the parallelogram BH, and the solid AL: AL shall be similar and similarly situated to CD. *12. 6. 22.5. Because, as EC to GC, so BA to AK, the sides about the equal angles ECG, BAK, are proportionals; therefore the parallelogram BK is similar to EG. +1 Def. 6. For the same reason the parallelogram KH is similar * K H * 24. 11. M to GF, and HB to FE: wherefore three parallelo- * PROP. XXVIII. THEOR. B. 11. 11 Def. If a solid parallelopiped be cut by a plane passing through See N. the diagonals of two of the opposite planes; it shall be cut into two equal parts. *9.11. * 16. 11. * 34. 1. * 24. 11. * C. 11. See N. Let AB be a solid parallelopiped, and DE, CF, the diagonals of the opposite parallelograms AH, GB, viz. those which are drawn betwixt the equal angles in each and because CD, FE, are each of them parallel to GA, and not in the same plane with it, CD, FE, are parallel; wherefore the diagonals CF, DE, are in the plane in which the parallels are, and are themselves * parallels: and the plane CDEF shall cut the solid AB into two equal parts. * * G Because the triangle CGF is equal* to the triangle CBF, and the triangle DAE to DHE; and that the parallelogram CA is equal* and similar to the opposite one BE, and the parallelogram GE to CH; therefore the prism contained by the two triangles CGF, DAE, and the three parallelograms, CA, GE, EC, is equal to the prism contained by the two triangles CBF, DHE, and the three parallelograms BE, CH, EC; because they are contained by the same number of equal and similar planes, alike situated, and none of their solid angles are contained by more than three plane angles. Therefore the solid AB is cut into two equal parts by the plane CDEF. Q. E. D. N. B. The insisting straight lines of a parallelopiped, mentioned in the next and some following propositions, are the sides of the parallelograms betwixt the base and the opposite plane parallel to it.' PROP. XXIX. THEOR. Solid parallelopipeds upon the same base, and of the same altitude, the insisting straight lines of which are terminated in the same straight lines in the plane opposite to the base, are equal to one another. Let the solid parallelopipeds, AH, AK, be upon the same base AB, and of the same altitude, and let their insisting straight lines AF, AG, LM, LN, be terminated in the same straight line FN, and CD, CE, BH, BK, be terminated in the same straight line DK: the solid AH shall be equal to the solid AK. First, let the parallelograms DG, HN, which are opposite to the base AB, have a common side HG. Then because the solid AH is cut by the plane AGHC passing through the diagonals, AG, CH, of the opposite planes ALGF, CBHD, AH is cut into * two equal parts by the plane AGHC; therefore the *28.11. solid AH is double of the prism which is contained betwixt the triangles ALG, CBH: for the same reason, because the solid AK is cut by the plane LGHB, through the diagonals LG, BH of the opposite planes N ALNG, CBKH, the solid AK is double of the same Next, let the parallelograms DM, EN, opposite to the base, have no common side. Then, because CH, CK, * are parallelograms, CB is equal to each of the opposite * 34. 1. sides DH, EK; wherefore DH is equal to EK: add, or take away the the triangle BHK, * * 38. 1, and the parallelogram DG is equal to the paral- *36.1. lelogram HN: for the same reason, the triangle * * AFG is equal to the triangle LMN: and the parallelogram CF is equal to the parallelogram BM, and *24.11. CG to BN; for they are opposite. Therefore the prism which is contained by the two triangles AFG, CDE, and the three parallelograms, AD, DG, GC, is equal to the * C. 11. prism contained by the two triangles LMN, BHK, and the three parallelograms, BM, MK, KL. If therefore the prism LMN, BHK, be taken from the solid of which the base is the parallelogram AB, and in which FDKN is the one opposite to it; and if from this same solid there be taken the prism AFG, CDE; the remaining solid, viz. the parallelopiped AH, is equal to the re- +3 Ax. maining parallelopiped AK. Therefore solid parallelopipeds, &c. Q. E. D. PROP. XXX. THEOR. Solid parallelopipeds upon the same base, and of the same See N. altitude, the insisting straight lines of which are not terminated in the same straight lines in the plane opposite to the base, are equal to one another. Let the parallelopipeds CM, CN, be upon the same base AB, and of the same altitude, but their insisting straight lines AF, AG, LM, LN, CD, CE, BH, BK not terminated in the same straight lines: the solids CM, CN shall be equal to one another. M H Produce FD, MH, and NG, KE, and let them meet one another in the points O, P, Q, R; and join AO, LP, BQ, CR. And because the plane LBHM is parallel to the opposite plane ACDF, and that the plane LBHM is that in which are the parallels LB, MHPQ, in which also is the figure BLPQ; and the plane ACDF is that in F which are the parallels AC, FDOR, in which also * LN; CR, CE, BQ, BK are in the same straight lines PROP. XXXI. THEOR. Solid parallelopipeds, which are upon equal bases, and of the same altitude, are equal to one another. Let the solid parallelopipeds AE, CF, be upon equal bases AB, CD, and be of the same altitude: the solid AE shall be equal to the solid CF. First, let the insisting straight lines be at right angles to the bases AB, CD, and let the bases be placed in the same plane, and so that the sides CL, LB may be in a straight line; therefore the straight line LM, which is at right angles to the plane in which the bases are, in the point L, is common to the two solids AE, CF: * 13. 11. let the other insisting lines of the solids be AG, HK, BE; DF, OP, CN: and first, let the angle ALB be equal to the angle CLD: then AL, LD are in a straight line*. Produce OD, HB, and let them meet in Q, 14. 1. and complete the solid parallelopiped LR, the base of which is the parallelogram LQ, and of which LM is one of its insisting straight lines. Therefore, because the parallelogram AB is equal to CD, as the base AB is to And *7.5. the base LQ, so is the base CD to the base LQ. because the solid parallelopided AR is cut by the plane LMEB, which is parallel to the opposite planes AK, DR; as the base AB is to the base LQ, so is the solid * 25. 11. AE to the solid LR: for the same reason, because the solid parallelopiped CR is cut by the plane LMFD, which is parallel to the opposite planes, CP, BR; as the base CD to the base LQ, so is the solid CF to the solid * P * R M JB AS HT LR: but as the base AB to the base LQ, so the base CD to the base LQ, as before was proved; therefore, as the solid AE to the solid LR †, so is the solid CF to † 11. 5. the solid LR: and therefore the solid AE is equal to 9. 5. the solid CF. * But let the solid parallelopipeds SE, CF, be upon equal bases SB, CD, and be of the same altitude, and let their insisting straight lines be at right angles to the bases; and place the bases SB, CD in the same plane, so that CL, LB may be in a straight line; and let the angles SLB, CLD be unequal: the solid SE shall be equal to the solid CF. Produce DL, TS until they meet in A, and from B draw BH parallel to DA; and let HB, OD produced meet in Q, and complete the solids AE, LR: therefore the solid AE is equal to the solid SE; because they are upon the same base LE, and of the same altitude, and their insisting straight lines, viz. LA, LS, BH, BT; MG, MV, EK, EX, are in * * 29.11. |