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PRO P. XV.
two straight lines which meet one another, but are not in the fame plane with the first two ; the plane which passes through these is parallel to the plane paffing through the others.
Let AB, BC, two straight lines meeting one another, be paa rallel to DE, EF that meet one another, but are not in the. fame plane with AB, BC: The planes through AB, BC, and DE, EF shall not meet, though produced.
From the point B draw BG perpendicular to the plane a 11. II. which pafses through DE, EF, and let it meet that plane in G; and through G draw GH parallel b to ED, and GK pa. b 31. I. sallel to EF : And because BG is perpendicular to the plane through DE, EF, it thall make right angles with every
F straight line meeting it in that B
G plane : But the straight lines
¢ 3. def. 11. C
K K CH, GK in that plane meet it: Therefore each of the angles BGH, BGK is a right angle : And becaufe BA is A
D parallel d to GH (for each of
de ir. them is parallel to DE, and they are not both in the fame plane with it) the angles GBA, BGH are together equal to two right angles : And BGH is a c 29. 1. right angle; therefore also GBA is a right angle, and GB per pendicular to BA; For the same reason, GB is perpendicular to BC: Since therefore the straight line GB stands at right angles to the two ftraight lines BA, BC, that cut one another in B; GB is perpendicular ' to the plane through BA, BC: And f 4. II. it is perpendicular to the plane through DE, EF; therefore BG is perpendicular to each of the planes through AB, BC, and DE, EF : But planes to which the same straight line is perpendicular, are parallel to one another : Therefore the plane through AB, 8 14. 11. BC is parallel to the plane through DE, EF. Wherefore, if two straight lines, &c. Q E. D).
PRO P. XVI.
IF two parallel planes be cut by another plane, their
common sections with it are parallels.
Let the parallel planes, AB, CD be cut by the plane EFHG, and let their common sections with it be EF, GH: EF is parallel to GH.
For, if it is not, EF, GH shall meet, if produced, either on
D meet one another ; but they do not meet, since they are parallel by the hypothesis :
G produced on the Gde of FH : In the same manner it may be proved, that EF, GH do not meet when produced on the side of EG: But straight lines which are in the same plane and do not meet, though produced either way, are parallel: Therefore EF is parallel to GH. Wherefore, if two parallel planes, &c, Q. E. D.
IF F two straight lines be cut by parallel planes, they shall
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 plane EBDX, the common sections
EX, BD, are parallela. For the same reason, because the two Book XI. parallel planes GH, KL are eut by the plane AXFC, the
H a 16.11.
1C common fections. AC, XF are A parallel : And because EX is G parallel to BD, a side of the triangle ABD, as AE to EB, so is b AX to XD. Again, be
b 2. 6. cause XF is parallel to AC, a fide of the triangle ADC, as
N fore c'; as AE to EB, so is CF
CII. . to FD. Wherefore, if two
M straight lines, &c. Q. E, D,
plane which passes through it fhall be at right angles to that plane.
Let the straight line AB be at right angles to a plane CK; every plane which pafses through AB shall be 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
a 3. dcf, its
; therefore AB is parallelo to FG. And AB is at right angles to b 28. I, the plane CK ; therefore FG is also at right angles to the same planec. But one plane is at right angles to another plane when c 8. II. ike straight lines drawn in one of the planes, at right angles
Book XI. to their common fection, are also at right angles to the other
plane d; and any straight line FG in the plane DE, which is at d 4. def. II.
right angles to CE the common seation of the planes, has been proved to be perpendicular to the other plane CK ; therefore the plane DE is at right angles to the plane CK. In 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.
IF two planes cutting one another be each of them per
pendicular to a third plane ; their common fection shall be perpendicular to the same plane.
Let the two planes AB, BC be each of them perpendicular to a third plane, and let BD be the common feètion of the first two ; BD is perpendicular to the third plane.
If it be not, from the point D draw, in the plane AB, the straight line DE at right angles to AD the common section of the plane AB with the third plane ; and in the plane BC draw DF at right angles to CD the common section of the plane BC with the third plane. And because the
section, DE is perpendicular to the third EF 3 4. def. 11. plane a. In the same manner, it may
be proved that DF is perpendicular to
angles to the third plane, upon the fame
fore, from the point D there cannot be
P O P
PRO P. XX. THE O R. a solid angle be contained by three plane angles, any See N. two of them 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
b 4. I. the base BE. And because BD, DC B
Е С are greater 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 than d. 25. 1. 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. Q. E. D.
C 20. Ia
VERY solid angle is contained by plain angles which