PROP. XV. THEOR. if two parallel planes be cut by a third plane, they have the same incli nation to that plane. Let AB and CD be two parallel planes, and EH a third plane cutting them : The planes AB and CD are equally inclined to EH. Let the straight lines EF and GH be the common section of the plane EH with the two planes AB and CD ; and from K, any point in EF, draw in the plane EH the straight line KM at right angles to EF, and let it meet GH in L; draw also KN at right angles to EF in the plane AB : and through the straight lines KM, KN, let a plane be made to pass, cutting the plane CD in the line LO. And because EF and GH are the common sections of the plane EH with the two parallel planes AB and CD, EF is parallel to GH (14. 2. Sup.). But EF is at right angles to the plane that passes through KN and KM (4, 2. Sup.), because it is at right angles to the lines KM and KN; therefore GH is also at right angles to the same plane (7. 2. Sup.), and it is therefore at right angles to theslines LM, LO which it meets in that plane. Therefore, since LM and LO are at right angles to LG, the common section of the two planes CD and EH, the angle OLM is the inclination of the plane CD to the plane EH (4. def. 2. Sup ). For the same reason the angle MKN is the inclination of the plane AB to the plane EH. But because KN and LO are parallel, being the common sections of the parallel planes AB and CD with a third plane, the interior angle NKM is equal to the exterior angle OLM (29. 1.); that is, the inclination of the plane AB to the plane EH, is equal to the inclination of the plane CD to the same plane FH. Therefore, &c. Q. E. D, PROP. XVI. THEOR. If two straight lines be cut by parallel planes, they must be cut in the same ratio. Let the straight lines AB, CD be cut by the parallel planes GH, KL, MN, in the points A, E, B ; C, F, D: As AE is to EB, so is CF to FD. Join AC, BD, AD, and let AD meet the plane KL in the point X ; and join EX, XF : Because the two parallel planes KL,MN are cut by the н plane EBDX, the common sections C EX, BD, are parallel (14. 2. Sup.). For the same reason, because the two parallel planes GH, KL are cut by the plane AXFC, the common sections AC, XF are parallel : And be. cause EX is parallel to BD, a side 1 of the triangle ABD, as AE to EB, E X 80 is (2. 6.) AX to XD. Again, be K cause XF is parallel AC, a side of the triangle ADC, as AX to XD,so is CF to FD : and it was proved that AX is to XD, as AE to EB : Therefore (11.5.), as AE to EB,so is CF to FD. Wherefore, if two straight lines,&c. B D Q. E. D. M PROP. XVII. THEOR. If a straight line be at right angles to a plane, every plane which passes through that line is at right angles to the first mentioned plane. Let the straight line AB be at right angles to a plane CK ; every plane wybich passes through AB is at right angles to the plane CK. Let any plane DE pass through AB, and let CE be the common section of the planes DE, CK ; take any point F in CE, from which draw FG in the plane DE at right angles to CE; And because AB is perpendicular to the plane CK, therefore D G A H it is also perpendicular to every straight line meeting it in that plane (1. def. 2. Sup.) ; and conse. quently it is perpendicular to CE: K Wherefore ABF is a right angle ; but GFB is likewise a right angle ; therefore AB is parallel (28. 1.) to FG. And AB is at right angles to the plane CK : therefore FG is F B E also at right angles to the same plane (7. 2. Sup.). But one plane is at right angles to another plane when the straight lines drawn in one of the planes, at right angles to their common section, are also at right angles to the other plane (def. 2. 2.); and any straight line FG in the plane DE, which is at right angles to CE, the common section of the planes, has been proved to be perpendicular to the other plane CK; therefore the plane DE is at righi ingles to the plane CK. la like manner, it may be proved that all the planes which pass through AB are at right angles to the plane CK. Therefore, if a straight line" &c. Q. E. D. PROP. XVIII. THEOR. If two planes cutting one another be each of them perpendicular to a third plane, their common section is perpendicular to the same plane. Let the two planes AB, BC be each of them perpendicular to a third plane, and BD be the common section of the first two ; BD is perpendicular to the plane ADC. From D in the plane ADC, draw DE perpendicular to AD, and DF to DC. Because DE is perpendicular to AD, the common section of the planes AB and ADC; and because the plane AB is at right angles to ADC, DE is at B right angles to the plane AB (def.2.2 Sup.), and tht refore also to the straight line BD in that plane (def. 1. 2. Sup.). For the same reason, DF is at right angles to DB. Since BD is therefore at right angles to both the lines DE and DF, it is at right angles to the plane in which DE and DF are, that is, to the plane ADC (4. 2. Sup.). Wherefore, &c. Q. E. D. Two straight lines not in the same plane being given in position, to draw straight line perpendicular to them both. Let AB and CD be the given lines, which are not in the same plane; it is required to draw a straight line which shall be perpendicular both to AB and CD. In AB take any point E, and through E draw EF parallel to CD, and let EG be drawn perpendicular to the plane which passes through EB, BF (10. 2. Sup.). Through AB and EG let a plane pass, viz. GK AB. K. and let this plane meet CD in H; from H draw HK perpendicular to AB ; and HK is the line required. Through H, draw HG parallel to Then, since HK and GE, which are in the same plane, are both at right angles to the straight line AB, they are parallel to one another. And because the lines HG, HD are parallel to the lines EB, EF, each to each, the plane GHD is parallel to the plane (13. 2. Sup.) BEF; and therefore EG, which is perpendicular to the plane BEF, is perpendicular also to the plane (Cor. 13. 2. Sup.)GHD. Therefore HK, wbich is parallel to GE, is also perpendicular to the plane GHD (7. 2. Sup,), and it is therefore perpendicular to HD (def. 1. 2. Sup.), which is in that plane, and it is also perpendicular to AB; therefore HK is drawn perpendicular to the two given lines, AB and CD. Which was to be done. PROP. XX. THEOR. If a solid angle be contained by three plane angles, any two of these; angles are greater than the third. Let the solid angle at A be contained by the three plane angles BAC, CAD, DAB. Any two of them are greater than the third. If the angles BAC, CAD, DAB be all equal, it is evident that any two of them are greater than the third. But if they are not, let BAC, be that angle which is not less than either of the other two, and is greater than one of them, DAB; and at the point A in the straight line AB, make in the plane which passes through BA, AC, the angle BAE equal (23. 1.) to the angle DAB; and make AE equal to AD, and through E draw BEC cutting AB, AC in the points B,C, and join DB, DC. And because DA is equal to AE, and AB is common to the C C two triangles ABD, ABE, and also the angle DAB equal to the angle EAB; therefore the base DB is equal D B 2 (4.1.) to the base BE. And because BD, DC are greater (20.1.) than CB, and one of them BD has been proved equal to BE, a part of CB, therefore the other DC is greater than the remaining part EC. And because DA is equal to AE, and AC common, but the base DC greater than the base EC; therefore the angle DAC is greater (25. 1.) than the angle EAC; and, by the construction, the angle DAB is equal to ;; the angle BAE ; wherefore the angles DAB, DAC are together greater than BAE, EAC, that is, than the angle BAC. But BAC is not less than either of the angles DAB, DAC; therefore BAC,with either of them, is greater than the other. Wherefore, if a solid angle, &c R.E.D. PROP. XXI. THEOR. The plane angles which contain any solid angla are together less than four right angles. a > Let A be a solid angle contained by any number of plane angles BAC CAD, DAE, EAF, FAB; these together are less than four right an gles Let the planes which contain the solid angle at A be cut by another plane, and let the section of them by that plane be the rectilineal figure BCDEF. Aod because the solid angle at B is contained by three plane angles CBA, ABF, FBC, of which any two are greater (20. 2. Sup.) than the third, the angles CBA, ABF are greater than the angle FBC: For the same reason, the two plane angles at each of B the points C, D, E, F, viz. the angles which are at the bases of the triangles having the common vertex A, are greater than the third angle at the same point, which is one of the angles of the figure T BCDEF : therefore all the angles at the bases of the triangles are together great D E er than all the angles of the figure : and because all the angles of the triangles are together equal to twice as many right angles as there are triangles (32. 1.); that is, as there are sides in the figure BCDEF ; and because all the angles of the figure, together with four right angles, are likewise equal to twice as many right angles as there are sides in the figure (cor. i. 32. 1.); therefore all the angles of the triangles are equal to all the angles of the rectilinéal figure, together with four right angles. But all the angles at the bases of the triangles are greater than all the angles of the rectilineal, has as been proved. Wherefore, the remaining angles of the triangles, viz. those at the vertex, which contain the solid angle at A, are less than four right angles. . Therefore every solid angle,&c. Q. E. D. Bb |