Book XI. PROP. C. THEOR. Solid figures contained by the same number of equal See N. and similar planes, alike situated, and having none of their solid angles contained by more than three plane angles, are equal and similar to one another. Let AG, KQ be two solid figures contained by the same number of similar and equal planes, alike situated, viz. let the plane AC be similar, and equal to the plane KM, the plane AF to KP; BG to LQ; GD to QN; DE to NO; and, lastly, FH similar, and equal to PR: the solid figure AG is equal, and similar to the solid figure KQ. Because the solid angle at A is contained by the three plane angles BAD, BAE, EAD, which, by the hypothesis, are equal to the plane angles LKN, LKO, OKN, which contain the solid angle at K, each to each ; therefore the solid angle at A is equal a to the solid angle at K: In the same manner, the a B. 11. other solid angles of the figures are equal to one another. If, then, the solid figure AG be applied to the solid figure KQ, first the plane figure AC being H G R Q applied to the O plane figure KM; P the straight line 0 с N M AB, coinciding with KL, the figure AC must А B K L coincide with the figure KM, because they are equal and similar: Therefore the straight lines AD, DC, CB coincide with KN, NM, ML, each with each ; and the points A, D, C, B, with the points K, N, M, L: And the solid angle at A coincides with the solid angle at K; wherefore the plane AF coincides with the plane KP, and the figure AF with the figure KP, because they are equal and similar to one another: Therefore the straight lines AE, EF, FB, coincide with KO, OP, PL; and the points E, F, with the points 0, P. In the same manner, the figure AH coincides with the figure KR, and the straight line DH with NR, and the point X with the point R: And because the solid angle at B is equal to the solid angle at L, it may be proved in the same manner, that the figure BG coincides with Book XI. the figure LQ, and the straight line CG with MQ, and the point G with the point Q : Since, therefore, all the planes and sides of the solid figure AG coincide with the planes and sides of the solid figure KQ, AG is equal and similar to KQ: And, in the same manner, any other solid figures whatever contained by the same 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. PROP. XXIV. THEOR. See N. If a solid be contained by six planes, two and two of which are parallel ; the opposite planes are similar and equal parallelograms. Let the solid CDGH be contained by the parallel planes AC, GF; BG, CE; FB, AE: Its opposite planes are similar and equal parallelograms. Because the two parallel planes BG, CE, are cut by the a 16. 11. plane AC, their common sections AB, CD are parallela. Again, because the two parallel planes BF, AE are cut by the B H A G C С. F CF, which meet one another, and are not in the same plane with the D E other two; wherefore they contain b 10. 11. equal angles ; the angle ABH is therefore equal to the angle DCF: And because AB, BH, are equal to DC, CF, and the angle ABH equal to the angle DCF; therefore the base AH is equal to the base DF, and the triangle ABH to the triangle DCF: and the parallelod 34. 1. gram BG is doubled the triangle ABH, and the parallelo gram CE double the triangle DCF; therefore the parallelogram BG is equal and similar to the parallelogram CE. In the same manner, it may be proved, that the parallelogram AC c 4. 1. is equal and similar to the parallelogram GF, and the paral- Book XI. lelogram AE to BF. Therefore, “ if a solid,” &c. Q. E. D. PROP. XXV. THEOR. If a solid parallelepiped be cut by a plane parallel See N. to two of its opposite planes ; it will be divided into two solids, which will be to one another as their bases. Let the solid parallelepiped ABCD be cut by the plane EV, which is parallel to the opposite planes AR, HD, and divides the whole into the two solids ABFV, EGCD; as the base AEFY of the first is to the base EHCF of the other, so 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 EA, 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 parallelo grams LO, KY, AF are equal a : And likewise the parallelo- a 36. 1. grams KX, KB, AG a: as also the parallelograms LZ, KP, b 24. 11. AR, because they are opposite planes: For the same reason, the parallelograms EC, HQ, MS are equal a ; and the parallelograms HG, HI, IN, as also b HD, MU, NT: Therefore three planes of the solid LP are equal and similar to three planes of the solid KR, as also to three planes of the solid AV: But the three planes opposite to these three are equal and similar to them in the several solids, and none of their solid angles are contained by more than three plane angles: Therefore the three solids PL, KR, AV are equal to one another: c c. 11. For the same reason, the three solids ED, HU, MT are equal Book XI. to one another: Therefore, what multiple soever the base LF is of the base AF, the same multiple is the solid LV of the solid AV: For the same reason, whatever multiple the base NF is of the base HF, the same multiple is the solid NV of the solid ED: And if the base LF be equal to the base NF, c C. 11. the solid LV is equal to the solid NV; and if the base LF be greater than the base NF, the solid LV is greater than the solid NV: and if less, less: Since then there are four magnitudes, viz. the two bases AF, FH, and the two solids AV, ED, and of the base AF and solid AV, the base LF and solid LV are any equimultiples whatever; and of the base FH and solid ED, the base FN and solid NV are any equimultiples whatever; and it has been proved, that if the base LF is greater than the base FN, the solid LV is greater than the d 5. def. 5. solid NV; and if equal, equal; and if less, less. Therefore, as the base AF is to the base FH, so is the solid AV to the solid ED. Wherefore, “ if a solid,” &c. Q. E. D. PROP. XXVI. PROB. See N. At a given point in a given straight line, to make a solid angle equal to a given solid angle contained by three plane angles. Let AB be a given straight line, A a given point in it, and D a given solid angle contained by 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. In the straight line DF, take any point F, from which draw a 11. 11. a FG perpendicular to the plane EDC, meeting that plane in G; 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; c 12. 11. then make AK equal to DG, and from the point K erectC KH b 23. 1. at right angles to the plane BAL; and make KH equal to Book XI. GF, and join AH: Then the solid angle at A, which is contained by the three plane angles BAL, BAH, HAL, is equal to the solid angle at D, contained by the three plane angles EDC, EDF, FDC. Take the equal straight lines AB, DE, and join HB, KB, FE, GE: And because FG is perpendicular to the plane EDC, it makes right angles d with every straight line meeting d 3. def. 11. it in that plane: Therefore each of the angles FGD, FGE, is a right angle: For the same reason, HKĀ, HKB are right angles : And because KA, AB, are equal to GD, DE, each to each, and contain equal angles, therefore the base BK is equale e 4. 1. to the base EG: And KH is equal to GF, and HKB, FGE are right angles, therefore HB is equal to FE: Again, because AK, KH are equal to DG, GF, and contain right angles, the base AH is equal to the base DF; and AB is equal to DE: therefore HA, AB are equal to FD, DE, and the base HB is equal to the base FE, therefore the angle BAH is equal to the angle f 8. 1. EDF: For the same A D reason, the angle HAL is equal to the angle FDC. Because if AL and DC be made equal, and B KL, HL, GC, FC be joined, since the K H E 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 o 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 solid angle at A, are equal to the three plane angles EDC, EDF, FDC, which contain the solid angle at D, each to each, and are situated in the same order, the solid angle at A is equal to the solid angle at D. Therefore, g B. 11. at a given point, in a given straight line, a solid angle has G |