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Book XI. Take in each of the straight lines AB, AC, AD any points
B, C, D, and join BC, CD, DB: Then, because the folid
angle at B is contained by the three plain angles CBA, ABD, * 20. II. DBC, any two of them are greater than the third ; there
fore the angles CBA, ABD are greater than the angle DBC:
fore the fix angles CBA, ABD, BCA, A
Next, Let the solid angle at A be contained by any number
Let the planes in which the angles are, be cut by a plane, and
ther greater than all the angles of the polygon : And because Book XI. all the angles of the triangles are together equal to twice as many right angles as there are triangles; that is, as there are b 32. I. fides in the polygon BCDEF: and that all the angles of the polygon, together with four right angles, are likewise equal to twice as many right angles as there are fides in the polygon; c I. Cor. therefore all the angles of the triangles are equal to all the an- 34• 1. gles of the polygon together with four right angles. But all the angles at the bases of the triangles are greater than all the angles of the polygon, as has 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.
Fevery two of three plain angles be greater than the see N.
third, and if the straight lines which contain them be all equal; a triangle may be made of the straight lines that join the extremities of those equal straight lines.
Let ABC, DEF, GHK be three plane angles, whereof e. very two are greater than the third, and are contained by the equal straight lines AB, BC, DE, EF, GH, HK; if their extremities be joined by the straight lines AC, DF, GK, a triangle may be made of three straight lines equal to AC, DF, GK; that is, every two of them are together greater than the third.
If the angles at B, E, H are equal; AC, DF, GK are also equal", and any two of them greater than the third : But * 4. to if the angles are not all equal, let the angle ABC be not less than either of the two at E, H; therefore the straight line AC is not less than either of the other two DF, GKD; and b 4.0r 24550 it is plain that AC, together with either of the other two, must be greater than the third : Also DF with GK are greater than AC: For, at the point B in the straight line AB make the c 33, 1.
Book XI. angle ABL equal to the angle GHK, and make BL equal to
one of the straight lines AB, BC, DE, EF, GH HK, and join AL, LC; Then because AB, BL are equal to GH, HK, and the angle ABL to the angle GHK, the base al is equal to the base GK: And because the angles at E, H are greater than the angle ABC, of which the angle at H is equal to ABL; therefore the remaining angle at E is greater than the angle LBC:
F G K
EF, and that the angle DEF is greater than the angle LBC, d 24. 1. the base DF is greater than the base LC: And it bas been
proved that GK is equal to AL, therefore DF and GK are c 20. 1. greater than AL and LC: But AL and LC are greater than
AC; much more than are DF and GK greater than AC. Where
fore every two of these straight lines AC, DF, GK are greater f 22. J. than the third ; and, therefore, a triangle may be made, the
fides of which shall be equal to AC, DF, GK. Q. E. D.
O make a folid angle which shall be contained by
three given plane angles, any two of them being greater than the third, and all three together less than four right angles.
Let the three given plane angles be ABC, DEF, GHK, any two of wbich are greater than the third, and all of them together lets than four right angles. It is required to make a solid angle contained by three plane angles equal to ABC, DEF, GHK, each to each.
From the straight lines containing the angles, cut off AB, Book XI, BC, DE, EF, GH, HK all equal to one another ; and join AC, DF, GK: 'Then a triangle may be made of three straight a 22. II.
F lines equal to AC, DF, GK. Let this be the triangle LMN, . 22. R so that AC be equal to LM, DF to MN, and GK to LN; and about the triangle LMN describe a circle, and find its centre X, which will either be within the triangle, or in one of its fides, or without it.
First, Let the centre X be within the triangle, and join LX, MX, NX: AB is greater than I.X : If not, AB must either be equal to, or less than LX; first, let it be equal : Then because AB is equal to LX, and that AB is also equal to BC, and LX to XM, AB and BC are equal to LX and XM, each to each ; and the base AC is, by construction, equal to the base LM; wherefore the angle ABC is equal to the angle LXMd: For the same reason, the angle DEF is equal to the d 8. I. angle MXN, and the angle GHK to the angle NXL: Therefore the
R three angles ABC, DEF, GHK are equal to the three angles LXM, MXN, NXL: But the three angles LXM, MXN, NXL are equal to four right angles; therefore also
e 2. Cor. the three angles ABC, DEF, GHK
d 8. I.
f 21. I.
Book XI. base AC, the angle LOM is equal to the angle ABC And
fore LO, OM fall within the triangle LXM; for, if they fell
Next, Let the centre X of the circle fall in one of the fides of the triangle, viz. in MN, and
possible t: Wherefore AB is not e.
But, let the centre X of the circle fall within the triangle LMN, and join LX, MX, NX. In this case likewise AB is greater than LX: If not, it is either equal to, or less than LX: First, let it be equal ; it may be proved in the same manner, as in the first case, that the angle ABC is equal to the angle MXL, and GHK 'to LXN; therefore the whole angle MXN is equal to the two angles, ABC, GHK: But ABC and GHK are together greater than the angle DEF; therefore also the angle MXN is greater than DEF. And because DE,