FE, join DC cutting AB in G. AB is harmonically divided in G and F. Since BC is parallel to FD, the angle BCG is equal to GDF and the vertically opposite angles at G are equal; therefore the triangles DGF, BGC are similar, and BC BG :: FD: FG But FE being parallel to BC, (Eucl. vi. 2.) AB .. ex æquali, AB or AB : BC: AF FE=FD. AF :: BG: FG. (20.) If a given finite straight line be harmonically divided, and from its extremities and the points of division lines be drawn to meet in any point, so that those from the extremities of the second proportional may be perpendicular to each other, the line drawn from the extremity of this proportional will bisect the angle formed by the lines drawn from the extremities of the other two. F E Let the straight line AB be divided harmonically in the points G and F, and let the lines AC, BC, GC, FC be drawn to any A point C so that GC may be perpendicular to CA, the angle BCF will be bisected by CG. Through G draw EGD parallel to CA meeting CF in D, then EG being parallel to AC, the triangles EGB, ACB are similar; as also the triangles ACF, DFG, hence But AB AC :: GB: GE .. (Eucl. v. 15.) BG GD:: BG: GE, and therefore GD = GE, and GC is common, and the angles at G are right angles, therefore the angle DCG GCE, and FCB is bisected by CG. = (21.) If a straight line be drawn through any point in the line bisecting a given angle, and produced to cut the sides containing that angle, as also a line drawn from the angle perpendicular to the bisecting line; it will be harmonically divided. Let the angle ABC be bi- E sected by the line BD, and through any point D in this line draw GDFE meeting the sides in G and F, and BE a perpendicular to BD in E; then will EG EF FD. GD: F B C For through D draw AC parallel to BE and therefore perpendicular to BD, then the angles ADB, CDB being right angles are equal, and ABD CBD and BD is common to the triangles ADB, CDB, .. AD= DC. But DC being parallel to EB, = EG: GD: EB: DC:: EB: AD :: EF: FD, since the triangles EFB and AFD are similar, .. EG: EF:: GD: FD. (22.) If from a given point there be drawn three straight lines forming angles less than right angles, and from another given point without them a line be drawn intersecting the others so as to be harmonically divided; then will all lines drawn from that point meeting the three lines be harmonically divided. From A let AB, AC, AD be drawn making each of the angles BAC, CAD less than a right angle, and from a given point E let EBD be drawn so as to be harmoni B H C D K cally divided in C and B; then will any other line EF be harmonically divided in G and H. Through G draw IK parallel to BD, But DC: CB :: DE: EB .. (Eucl. v. 15.) DE : EB :: KG : GI and alt. DE : KG :: EB: GI and since DE is parallel to GK, (Eucl. vi. 2.) DE: KG:: EF: FG and EB being parallel to GI, :. EB : GI :: EH : HG, whence (Eucl. v. 15.) EF: FG:: EH: HG and alt. EF: EH:: FG: HG. (23.) If a straight line be divided into two equal, and also into two unequal parts, and be produced, so that the part produced may have to the whole line so produced the same ratio that the unequal segments of the line have to each other, then shall the distances of the point of unequal section from one extremity of the given line, from its middle point, from the extremity of the part produced, and from the other extremity of the given line, be proportionals. Let AB be divided into two equal E BD C A parts in Cand into two unequal parts in D, and produced to E, so that BE: EA:: BD: DA; then will AD: DC:: ED: DB. For since BE: EA:: BD : DA inv. AE: EB:: AD: DB div. AB BE :: 2 CD: DB : and AC: BE:: CD: DB alt. AC: CD:: BE: BD .. comp. AD: DC:: ED: DB. COR. The converse may easily be proved to be true. (24.) Three points being given; to determine another, through which if any straight line be drawn, perpendiculars upon it from two of the former, shall together be equal to the perpendicular from the third. Let A, B, C be the three given points. Join AB and bisect it in D. Join CD, from which cut off DE equal to a third part of it. E is the point required. D B F E H Through E, let any line FG be drawn, and let fall on it the perpendiculars AI, BG, DH, CF; then the angles at F and H being right angles, and the vertical angles at E equal, the triangles CFE, DHE are equiangular, .. FC: DH :: CE : ED :: 2 : 1, .. FC=2DH; but since AI, BG, DH are parallel, and AD=DB, .. AI+BG = 2 DH=FC. (25.) From a given point in one of two straight lines given in position, to draw a line to cut the other, so that if from the point of intersection a perpendicular be let fall upon the former, the segment intercepted between it and the given point, together with the first drawn line, may be equal to a given line. Let AB, BC be the lines given in position, and A the given point. Draw AD perpendicular to AB, and meeting BC in D; draw DE parallel to AB and equal to the given line. And draw EF parallel to AD, meeting CB in F. Join FA and produce it, and from D draw DG=DE, meeting FG in G, and draw AH parallel to DG, and let fall the perpendicular HI; AH and AI together are equal to the given line. Through Hdraw KL parallel to DE; then since GD is parallel to AH and HL to DE, .. DG : AH :: FD : FH :: DE : HL, but DG DE, .. AH= HL, = .. AH+AI=KL=DE = the given line. |