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Book XI. with the figure LQ, and the straight line CG with MQ, and
the point G with the point Q. fiace therefore all the planes and sides of the solid figure AG coincide with the planes and sides of the folid figure KQ, AG is equal and similar to KQ. and in the fame manner, any other solid figures whatever contained by the fame number of equal and similar planes, alike situated, and having none of their solid angles contained by more than three plane angles, may be proved to be equal and similar to one another. Q. E D.
PRO P. XXIV. THEO R.
a solid be contained by fix planes, two and two of
which are parallel ; the opposite planes are similar and equal parallelograms.
Iet the folid CDGH be contained by the parallel planes AC, CF; BG, CE; FB, A E. its oppoate planes are fuzilar and equal parallelograms.
Becau'e the two parallel planes, BG, CE are cut by the plane R. 16.11. AC, their common fections AB, CD are parallel *. again, because
the two parallel plores BF, AE are cut by the plane AC, their common fections AD, EC re parallel'. and AB is parallel to CD; tervore AC is a parallelocidin in
H lihe manne, it may be proven that each of the figures CE, TG, Gb, BP, AE is a parallelogram. join AH, DI'; and because AB is parallel to DC, and BH to CF; the two straight lines AB,
E BH, which meet one another, are parallel to DC and CF which meet one
another and are not in the same plane b.19.14. with the other two; wherefore they contain equal angles b; the
angle ABH is therefore equal to the angle DCF. and because AB, EH are equal to DC, CF, and the angle ABH equal to the angle DCF, therefore the base AH is equal to the bafe DF, and the
triangle ABH to the triangle DCF. and the parallelogram BG is d. 34. I.
double d of the triangle ABH, and the parallelogram CE double of the triangle DCF; therefore the parallelogram BG is equal and similar to the parallelogram CE. in the fame manner, it may be proved that the parallelogram AC is equal and similar to the pa
rallelogram GF, and the parallelogram AE to BF. Therefore if a Book XI. folid, &c. Q. E. D.
PRO P. XXV. THE O R.
two of its opposite planes ; it divides the whole
Let the folid parallelepiped ABCD be cut by the plane EV which is parallel to the opposite planes AR, HD, and divides the whole into the two folids ABFV, EGCD; as the base AEFY of the first is to the base EHCF of the other, fo is the solid ABFV to the solid EGCD.
Produce AH both ways, and take any number of straight lines HM, MN each equal to EH, and any number AK, KL each equal to LA, and complete the parallelograms LO, KY, HQ, MS, and the solids LP, KR, HU, MT. then because the straight lines LK, KA, AE are all equal, the parallelograms LO, KY, AF are equal a. a. 36. 5.
ន and likewise the parallelograms KX, KB, AG"; as also b the pa- b. 24. 11. rallelograms LZ, KP, AR, because they are opposite planes. for the same reason, the parallelograms EC, HQ, MS are equal“; and the parallelograms HG, HI, IN, as also b HD, MU, NT. therefore three plancs of the solid LP, are equal and similar to three planes of the folid KR, as also to three planes of the folid AV. but. the three planes opposite to these three are equal and similar to them in the several solids, and none of their folid angles are coiltained by more than three plane angles. therefore the three folds LP, KR, AV are equal to one another. for the same reason, the c. C. 11. three solids ED, HU, MT are equal to one another. thercfore what
Book XI. multiplc soever the base LF is of the base AF, the fame multiple is
the solid LV of the solid AV. for the same reason, whatever multiple the base NF is of the base HF, the fame multiple is the folid
NV of the folid ED. and if the bafe LF be equal to the bafe NF, c. C. 11. the folid LV is equal to the folid NV ; and if the base LF be
greater than the base NF, the folid LV is greater than the solid NV, and if lefs, less. since then there are four magnitudes, viz.
the two bafes AF, FH, and the two folids AV, ED, and of the base AF and folid AV, the bife LF and fold LV arc any equimultiples whatever; and of the base FH and folid ED, the base FN and folid NV are any equimultiples whatever ; and it has been proved, that if the base LF is greater than the base FN, the folid
LV is greater than the solid NV; and if equal, equal; and if less, d. s. Def.s. less. Therefore d as the base AF is to the base FH, fo is the solid
AV to the folid ED. Wherefore if a folid, &c. Q. E. D.
PRO P. XXVI. PROB.
solid angle equal to a given folid angle contained by three plane angles.
Let AB be a given straight line, A a given point in given solid angle contained by the three plane angles EDC, EDF, FDC. it is required to make at the point A in the straight line
AB a solid angle equal to the solid angle D. a. 11. 11d. In the straight line DF take any point F, from which draw
FG perpendicular to the plane EDC, meeting that plane in G; 6. 43. So join DG, and at the point A in the straight line AB make the
angle BAL equal to the angle EDC, and in the plane BAL make the angle BAK equal to the angle EDG; then make AK equal
to DG, and from the point K erect KH at right angles to the Book XI.
Take the equal straight lines AB, DE, and join HB, KB, FE,
A angle BAH is equalf to
f. 8. 1. the angle EDF. for the fame reafon, the angle HAL is equal to the B angle FDC. because if AL and DC be made K
I 죄 equal, and KL, HL, GC, FC be joined, since the whole angle BAL is equal to the whole EDC, and the parts of them BAK, EDG are, by the construction, equal; therefore the remaining angle KAL is equal to the remaining angle GDC. and because KA, AL are equal to GD, DC, and contain equal angles, the base KL is equal to the base GC. and KH is equal to GF, so that LK, KH are equal to CG, GF, and they contain right angles; therefore the base HL is equal to the base FC. again, because HA, AL are equal to FD, DC, and the base HL to the base FC, the angle HAL is equal to the angle FDC. therefore because the three plane angles BAL, BAH, HAL which contain the folid angle at A, are equal to the three plane angles EDC, EDF, FDC which contain the folid angle at D, each to each, and are situated in the same order ; the folid angle at A is equal to the folid angle at D. Therefore at a given z. B. 11.
Book XI. point in a given straight line a solid angle has been made equal to a
given solid angle contained by three plane angles. Which was
PROP. XXVII. PROB.
O describe from a given straight line a solid pa
rallelepiped similar, and similarly situated to one given.
2. 26. II.
b. 12. 6.
C. 22. s.
Let AB be the given straight line, and CD the given solid pa-
At the point A of the given straight line AB make a folid
AL are fimilar to three of the folid CD; and the three opposite d. 24. 11.
ones in each solid are equal d and similar to these, each to cach.
the figures are equal, each to each, and situated in the same order, e. B. 11. the solid angles are equal", each to each. Therefore the solid 1.11. Def.11. AL is funilar f to the folid CD. wherefore from a given straight
line AB a solid parallelepiped AL has been described similar, and