To draw a straight line at right angles to a given straight line, from a given point in the same. Let AB be a given straight line, and C a point given in it; See N. it is required to draw a straight line from the point C at right angles to AB. Take any point D in AC, and make CE equal to CD, and a 3. 1. upon DE describe the equi b lateral triangle DFE, and join FC; the straight line FC drawn from the given point C is at right angles to the given straight line AB. AD F C E B b 1. 1. Because DC is equal to CE, and FC common to the two triangles DCF, ECF; the two sides DC, CF, are equal to the two EC, CF, each to each; and the base DF is equal to the base EF; therefore the angle DCF is equal to the c 8. 1. angle ECF; and they are adjacent angles. But when the adjacent angles which one straight line makes with another straight line are equal to one another, each of them is called C 1. a right angle; therefore each of the angles DCF, ECF, is a d 10. Def. right angle. Wherefore from the given point C, in the given straight line AB, FC has been drawn at right angles to AB. Which was to be done. COR. By help of this problem, it may be demonstrated, that two straight lines cannot have a common segment. If it be possible, let the two straight lines ABC, ABD have the segment AB common to both of them. From the point B draw BE at right angles to AB; and because ABC is a straight To draw a straight line perpendicular to a given straight line of an unlimited length, from a given point without it. Let AB be the given straight line, which may be produced to any length both ways, and let C be a point without it. It is required to draw a straight line perpendicular to AB from Take any point D upon the C E b 3. Post. describe the circle EGF meeting AB in F, G; and bi H c 10. 1. sect FG in H, and join CF, A F CH, CG; the straight line D CH, drawn from the given 1. e 8. 1. point C, is perpendicular to the given straight line AB. Because FH is equal to HG, and HC common to the two triangles FHC, GHC, the two sides FH, HC are equal to the e d 15. Def. two GH, HC, each to each; and the base CF is equal to the base CG; therefore the angle CHF is equal to the angle CHG; and they are adjacent angles; but when a straight line standing on a straight line makes the adjacent angles equal to one another, each of them is a right angle; and the straight line which stands upon the other is called a perpendicular to it; therefore from the given point C a perpendicular CH has been drawn to the given straight line AB. Which was to be done. PROP. XIII. THEOR. The angles which one straight line makes with another upon the one side of it, are either two right angles, or are together equal to two right angles. Let the straight line AB make with CD, upon one side of Book I. it, the angles CBA, ABD; these are either two right angles, or are together equal to two right angles. For if the angle CBA be equal to ABD, each of them is a a b с right angle; but, if not, from the point B draw BE at right a Def. 10. angles to CD; therefore the angles CBE, EBD are two right b 11. 1. angles; and because CBE is equal to the two angles CBA, ABE together, add the angle EBD to each of these equals; therefore the angles CBE, EBD, are equal to the three angles c 2. Ax. CBA, ABE, EBD. Again, because the angle DBA is equal to the two angles DBE, EBA, add to these equals the angle ABC, therefore the angles DBA, ABC are equal to the three angles DBE, EBA, ABC; but the angles CBE, EBD have been demonstrated to be equal to the same three angles; and things that are equal to the same are equal to one another; d 1. Ax. therefore the angles CBE, EBD are equal to the angles DBA, ABC; but CBE, EBD are two right angles; therefore DBA, ABC are together equal to two right angles. Wherefore, "when a straight line," &c. Q. E. Ď. PROP. XIV. THEOR. If, at a point in a straight line, two other straight lines upon the opposite sides of it, make the adjacent angles, together equal to two right angles, these two straight lines shall be in one and the same straight line. At the point B in the straight line AB, let the two straight lines BC, BD upon the opposite sides of AB, make the adjacent angles ABC, ABD equal together to two right angles. BD is in the same straight line with CB. For, if BD be not in the same straight line with CB, let A E C B D 16 a 13. 1. Book I. BE be in the same straight line with it; therefore, because the straight line AB makes angles with the straight line CBE, upon one side of it, the angles ABC, ABE are together equal a to two right angles; but the angles ABC, ABD are likewise together equal to two right angles; therefore the angles CBA, ABE are equal to the angles CBA, ABD: Take away the common angle ABC, the remaining angle ABE is equal to the remaining angle ABD, the less to the greater, which is impossible; therefore BE is not in the same straight line with BC. And, in like manner, it may be demonstrated, that no other can be in the same straight line with it but BD, which therefore is in the same straight line with CB. Wherefore, b 3. Ax. 66 if at a point," &c. Q. E. Ď. PROP. XV. THEOR. If two straight lines cut one another, the vertical, or opposite, angles shall be equal. Let the two straight lines AB, CD, cut one another in the point E; the angle AEC shall be equal to the angle DEB, and CEB to AED. a a C E B D Because the straight line AE makes with CD the angles CEA, AED, these angles are a 13. 1. together equal to two right angles. Again, because the straight line DE makes with AB the angles AED, DEB, A these also are together equal to two right angles; and CEA, AED have been demonstrated to be equal to two right angles, wherefore the angles CEA, AED are equal to the angles AED, DEB. Take away the common angle AED, and the remaining angle CEA is equal to the remaining angle DEB. In the same manner it can be demonstrated, that the angles CEB, AED are equal. Therefore, if two straight lines," &c. Q. E. D. b 3. Ax. COR. 1. From this it is manifest, that if two straight lines cut one another, the angles which they make at the point where they cut, are together equal to four right angles. COR. 2. And consequently that all the angles made by any number of straight lines meeting in one point, are together equal to four right angles. PROP. XVI. THEOR. If one side of a triangle be produced, the exterior angle is greater than either of the interior opposite angles. Let ABC be a triangle, and let its side BC be produced to D, the exterior angle ACD is greater than either of the interior opposite angles CBA, BAC. Bisecta AC in E, join BE and produce it to F, and make EF equal to BE; join also FC, and produce AC to G. Because AE is equal to EC, and BE to EF; AE, EB are equal to CE, EF, A Book I. a 10. 1. E each to each; and the angle AEB is equal to the angle B CEF, because they are b 15. 1. opposite vertical angles; therefore the base AB is equal to the base CF, and c 4. 1. the triangle AEB to the triangle CEF, and the remaining angles to the remaining angles, each to each, to which the equal sides are opposite; wherefore the angle BAE is equal to the angle ECF; but the angle ECD is greater than the angle ECF; therefore the angle ACD is greater than BAE: In the same manner, if the side BC be bisected, it may be demonstrated that the angle BCG, that is, the angle ACD is d 15. l. greater than the angle ABC. Therefore," if one side," &c. Q. E. D. d PROP. XVII. THEOR. Any two angles of a triangle are together less than two right angles. Let ABC be any triangle; any two of its angles together are less than two right angles. Produce BC to D; and because ACD is the exterior angle of the triangle ABC, ACD is greater than the interior and opposite angle ABC; to each of these add the angle ACB; therefore the angles ACD, |