THE ELEMENTS OF EUCLID. BOOK XI. DEFINITIONS. I. II. III. when it makes right angles with every straight line meeting it IV. in one of the planes perpendicularly to the common section of V. contained by that straight line and another drawn from the VI. by two straight lines drawn from any the same point of their VII. Book XI. Two planes are said to have the same, or a like inclination to one another, which two other planes have, when the said angles of inclination are equal to one another. VIII. Parallel planes are such which do not meet one another though produced. IX. A solid angle is that which is made by the meeting of more than See N. two plane angles, which are not in the same plane, in one point. X. " The tenth definition is omitted for reasons given in the notes.' See N. XI. See N. Similar solid figures are such as have all their solid angles equal, each to each, and which are contained by the same namber of XII. tuted betwixt one plane and one point above it in which they XIII. that are opposite are equal, similar, and parallel to one ano- XIV. A sphere is a solid figure described by the revolution of a semicircle about its diameter, which remains unmoved. XV. XVI. XVII. through the centre, and is terminated both ways by the super- XVIII. . A cone is a solid figure described by the revolution of a right angled triangle about one of the sides containing the right angle, which side remains fixed. If the fixed side be equal to the other side containing the right angle, the cone is called a right angled cone ; if it be less than the other side, an obtuse angled, and if greater, an acute angled cone. Book XI. XIX. The axis of a cone is the fixed straight line about which the tri. angle revolves. XX. XXI. angled parallelogram about one of its sides, which remains XXII. The axis of a cylinder is the fixed straight line about which the parallelogram revolves. XXIII. The bases of a cylinder are the circles described by the two revolving opposite sides of the parallelogram. XXIV. XXV. XXVI. XXVII. An octahedron is a solid figure contained by eight equal and equilateral triangles. XXVIII. A dodecahedron is a solid figure contained by twelve equal pentagons which are equilateral and equiangular. XXIX. DEF. A. figures, whereof every opposite two are parallel. Book XI. PROP. I. THEOR. ONE part of a straight line cannot be in a plane See N. and another part above it. If it be possible, let AB, part of the straight line ABC, be in the plane, and the part BC above it : and since the straight line AB is in the plane, it can be produced in that plane : let it be pro С duced to D: and let any plane pass through the straight line AD, and A be turned about it until it pass А D through the point C; and because the points B, C are in this plane, the straight line BC is in it a : therefore there are two a 7. def.i. straight lines ABC, ABD in the same plane that have a common segment AB, which is impossible b. Therefore, one part, &c. b Cor.11. Q. E. D. 1. PROP. II. THEOR. . TWO straight lines which cut one another are in See N one plane, and three straight lines which meet one another are in one plane. Let two straight lines AB, CD cut one another in E; AB, CD are one plane: and three straight lines EC, CB, BE which meet one another, are in one plane. Let any plane pass through the straight A D line EB, and let the plane be turned about EB, produced, if necessary, until it pass through the point C: then because the points E, C are in this plane, the straight E line EC is in it a : for the same reason, the a 7. def.1. straight line BC is in the same ; and, by the hypothesis, EB is in it: therefore the three straight lines EC, CB, BE are in one plane : but in the plane in which EC, EB CA B are, in the same are b CD, AB: therefore, b 1. 11 AB, CD are in one plane. Wherefore, two straight lines, &c. Q. E. D. X Book XI. PROP. III. THEOR. See N. IF two planes cut one another, their common sec. tion is a straight line. Let two planes AB, BC, cut one another, and let the line DB B ch A be a straight line. Wherefore, if two D planes, &c. Q. E. D. F PROP. IV. THEOR. See N. IF a straight line stand at right angles to each of two straight lines in the point of their intersection, it shall also be at right angles to the plane which passes through them, that is, to the plane in which they are. Let the straight line EF stand at right angles to each of the straight lines AB, CD in E, the point of their intersection: EF is also at right angles to the plane passing through AB, CD. Take the straight lines AE, EB, CE, ED all equal to one another; and through E draw, in the plane in which are AB, CD, any straight line GEH; and join AD, CB; then, from any point F in EF, draw FA, FG, FD, FC, FH, FB: and because the two straight lines AE, ED are equal to the two BE, EC, and a 15. 1. that they contain equal angles a AED, BEC, the base AD is b 4.1. equal b to the base BC, and the angie DAE to the angle EBC: and the angle AEG is equal to the angle BEH a ; therefore the triangles AEG, BEH have two angles of one equal to two angles of the other, each to each, and the sides AE, EB, aujacent to the equal angles, equal to one another; wherec 26. 1. fore they shall have their other sides equalc: GE is therefore |