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certain limits, and that certain magnitudes increase while others decrease within those limits; and after having reached a certain value, the former begin to decrease, while the latter increase. This circumstance gives rise to questions of marima and minima, or the greatest and least values which certain magnitudes may admit of in indeterminate problems.
In the following collection of problems and theorems, most will be found to be of so simple a ch cier, (being almost obvious deductions from propositions in the Elements) as scarcely to admit of the principle of the Geometrical Analysis being applied, in their solution.
It must however be recollected that a clear and exact knowledge of the first principles of Geometry must necessarily precede any intelligent application of them. Indistinctness or defectiveness of understanding with respect to these, will be a perpetual source of error and confusion. The learner is therefore recommended to understand the principles of the Science, and their connexion, fully, before he attempt any applications of them. The following directions may assist him in his proceedings.
ANALYSIS OF THEOREMS. 1. Assume that the Theorem is true.
2. Proceed to examine any consequences that result from this admission, by the aid of other truths respecting the diagram, which have been already proved.
3. Examine whether any of these consequences are already known to be true, or to be false.
4. If any one of them be false, we have arrived at a reductio ad absurdum, which proves that the theorem itself is false, as in Euc. 1. 25.
5. If none of the consequences so deduced be known to be either true or false, proceed to deduce other consequences from all or any of these, as in (2).
6. Examine these results, and proceed as in (3) and (4); and if still without any conclusive indications of the truth or falsehood of the alleged theorem, proceed still further, until such are obtained.
ANALYSIS OF PROBLEMS. 1. In general, any given problem will be found to depend on several problems and theorems, and these ultimately on some problem or theorem in Euclid.
2. Describe the diagram as directed in the enunciation, and suppose the solution of the problem effected.
3. Examine the relations of the lines, angles, triangles, &c. in the diagram, and find the dependence of the assumed solution on some theorem or problem in the Elements.
4. If such cannot be found, draw other lines parallel or perpendicular as the case may require, join given points, or points assumed in the solution, and describe circles if need be: and then proceed to trace the dependence of the assumed solution on some theorem or problem in Euclid.
5. Let not the first unsuccessful attempts at the solution of a Problem be considered as of no value ; such attempts have been found to lead to the discovery of other theorems and problems.
PROPOSITION I. PROBLEM.
To trisect a given straight line. ANALYSIS. Let AB be the given straight line, and suppose i. divided into three equal parts in the points D, E.
On DE describe an equilateral triangle DEF,
then DF is equal to AD, and FE to EB. On AB describe an equilateral triangle ABC,
and join AF, FB. Then because AD is equal to DF, therefore the angle AFD is equal to the angle DAF, and the two angles DAF, DFA are double of one of them DAF.
But the angle FDE is equal to the angles DAF, DFA,
and the angle FDE is equal to DAC, each being an angle of an El
wherefore the angle DAC' is bisected by AF.
and the angle FAD to DFA;
and consequently FD is parallel to AC. Synthesis. Upon AB describe an equilateral triangle ABC, bisect the angles at A and B by the straight lines
AF, BF, meeting in F; through F draw FD parallel to AC, and FE parallel to Bi.
Then AB is trisected in the points D, E.
but the angle FAD is equal to the angle FAC,
and therefore DF is equal to DA.
and FED to CBA; (I. 29.)
ACB. Hence the three sides of the triangle DFE are equal to one another,
and DF has been shewn to be equal to DA,
therefore AD, DE, EB are equal to one another. Hence the following theorem.
If the angles at the base of an equilateral triangle be bisected by two lines which meet at a point within the triangle; the two lines drawn from this point parallel to the sides of the triangle, divide the base into three equal parts.
Note. There is another method whereby a line may be divided into three equal parts :—by drawing from one extremity of the given line, another making an acute angle with it, and taking three equal distances from the extremity, then joining the extremities, and through the other two points of division, drawing lines parallel to this line through the other two points of division, and to the given line;
the three triangles thus formed are equal in all respects. This may be extended for any number of parts, and is a particular case of Euc. VI. 10.
PROPOSITION II. THEOREM. If two opposite sides of a parallelogram be bisected, and two lines be draron from the points of bisection to the opposite angles, these two lines trisect the diagonal. Let ABCD be a parallelogram of which the diagonal is AC.
Let AŘ be bisected in E, and DC in F,
Then AC is trisected in the points G, HI.
Through E draw EK parallel to AC and meeting FB in K.
therefore EB is equal to DF; and these equal and parallel straight lines are joined towards the
same parts by DĒ and FB;
And because AEB meets the parallels EK, AC, therefore the exterior angle BEK is equal to the interior angle EAG. For a similar reason, the angle EBR is equal to the angle AEG.
Hence in the triangles AEG, EBK, there are the two angles GAE, AEG in the one, equal to the two angles KEB, EBK in the other, and one side adjacent to the equal angles in each triangle, namely AE equal to EB;
therefore AG is equal to EK, (I. 26.) but EK is equal to GH, (1. 34.) therefore AG is equal to GH. By a similar process, it may be shewn that GH is equal to HC.
Hence AG, GH, HC are equal to one another,
and therefore AC is trisected in the points G, H.
PROPOSITION III. PROBLEM. Draw through a given point, between two straight lines not parallel, a straight line which shall be bisected in that point.
Analysis. Let BC, BD be the two lines meeting in B, and let A be the given point between them.
Suppose the line EAF drawn through A, so that EA is equal to AF,
through A draw AG parallel to BC, and GH parallel to EF. Then ĂGHE is a parallelogram, wherefore AE is equal to GH, but EA is equal to AF by hypothesis; therefore GH is equal to AF.
Hence in the triangles BİG, GAF,
also the side GH is equal to AF;
therefore BG is equal to GF. Synthesis. Through the given point A, draw AG parallel to BC,
on GD, take GF equal to GB;
and produce FA to meet BC in E;
draw GH parallel to AE. Then in the triangles BGH, ĜFA, the side BG is equal to GF, and the angles GBH, BGH are respectively equal to FGA, GFA;
wherefore GH is equal to AF, (1. 26.)
but GH is equal to AE, (1. 34.)
PROPOSITION IV. PROBLEM. From two given points on the same side of a straight line given in position, draw two straight lines which shall meet in that line, and make equal angles with it; also prove, that the sum of these two lines is less than the sum of any other two lines drawn to any other point in the line.
Analysis. Let A, B be the two given points, and CD the given line. Suppose G the required point in the line, such that AG and BG being joined, the angle AGC' is equal to the angle BGD.
Draw AF perpendicular to CD and meeting BG produced in E. Then, because the angle BGD is equal to AGF, (hyp.)
and also to the vertical angle FGE, (1. 15.) therefore the angle AGF is equal to the angle EGF;
also the right angle AFG is equal to the right angle EFG, and the side FG is common to the two triangles AFG, EFG,
therefore AG is equal to EG, and AF to FE. Hence the point E being known, the point G is determined by the intersection of CD and BĚ.
Synthesis. From A draw AF perpendicular to CD, and produce it to E, making FE equal to AF, and join BE cutting CD in G.
Join also ÀG. Then AG and BG make equal angles with CD. For since AF is equal to FE, and FG is common to the two triangles AGF, EGF, and the included angles AFG, EFG are equal;
therefore the base AG is equal to the base EG,
and the angle AGF to the angle EGF,
therefore the angle AGF is equal to the angle BGD; that is, the straight lines AG and BG make equal angles with the straight line CD.
Also the sum of the lines AG, GB is a minimum.
greater than the third side, therefore EH, HB are greater than EB in the triangle EHB.
But EG is equal to AG, and EH to AH;
therefore AH, HB are greater than AG, GB. That is, AG, GB are less than any other two lines which can be drawn from A, B, to any other point H in the line CD.
By means of this Proposition may be found the shortest path from one given point to another, subject to the condition, that it shall meet two given lines.
PROPOSITION V. PROBLEM. Given one angle, a side opposite to it, and the sum of the other two sides, construct the triangle.
Analysis. Suppose BAC the triangle required, having BC equal to the given side, BAC equal to the given angle opposite to BC, also BD equal to the sum of the other two sides.
Join DC. Then since the two sides BA, AC are equal to BD, by taking BA from these equals, the remainder AC is equal to the remainder AD.
Hence the triangle ACD is isosceles, and therefore the angle ADC is equal to the angle ACD.
But the exterior angle BAC of the triangle ADC is equal to the two interior and opposite angles ACD and ADC:
Wherefore the angle BAC is double the angle BDC, and BDC is the half of the angle BAC.
Hence the synthesis.