« ΠροηγούμενηΣυνέχεια »
Then every circle which has its centre in AE, and passes through P, must also pass through P'.
Ex. 1, p. 233. Hence the problem is now reduced to drawing a circle through P and P to touch either AC or AB.
Ex. 21, p. 253. Produce P'p to meet AC at S. Describe a square equal to the rect. SP, SP'; and cut off ŚR equal to a side of the square. Describe a circle through the points P', P, R. Then since the rect. SP, SP'=the sq. on SR, Constr. .. this circle touches AC at R;
III. 37, and since its centre is in AE, the bisector of the L BAC, it may be shewn also to touch AB.
Q.E.F. NOTES. (i) Since SR may be taken on either side of S, it follows that there will be two solutions of the problem.
(ii) If the given straight lines are parallel, the centre lies on the parallel straight line mid-way between them, and the construction proceeds as before.
24. To describe a circle to touch two given straight lines and a given circle. Let AB, AC be the two given
H, st. lines, and D the centre of the given circle. It is required to describe a circle to touch AB, AC and the circle
G whose centre is D.
OF VR Draw EF, GH par to AB and AC respectively, on the sides
B remote from D, and at distances from them equal to the radius E
M of the given circle.
Describe the O MND to touch EF and GH at M and N, and to pass through D.
Ex. 23, p. 254. Let O be the centre of this circle. Join OM, ON, OD meeting AB, AC, and the given circle at P, Q, and R.
Then a circle described with centre O and radius OP will touch
Constr. OP=OQ=OR. .. a circle described with centre O, and radius OP, will pass through Q and R. And since the _s at M and N are rt. angles,
III. 18. :: the _s at P and Q are rt. angles ;
1. 29. the O PQR touches AB and AC.
And since R, the point in which the circles meet, is on the line of centres OD,
:. the O PQR touches the given circle. Q.E.F. NOTE. There will be two solutions of this problem, since two circles may be drawn to tcuch EF, GH and to pass through D.
25. To describe a circle to pass through a given point and touch a given straight line and a given circle.
Let P be the given point, AB the given st. line, and DHE the given circle, of which C is the centre. It is required to describe a circle to pass through P, and to touch AB and the O DHE.
Through C draw DCEF perp. to
B and by describing a circle through F, E, and P, find a point K in DP (or DP produced) such that the rect. DE, DF = the rect. DK, DP.
Describe a circle to pass through P, K, and touch AB: Ex. 21, p. 253.
III. 31. also the angle at F is a rt. angle;
Constr. :. the points E, F, G, H are concyclic : .. the rect. DE, DF =the rect. DH, DG:
III. 36. but the rect. DE, DF=the rect. DK, DP: Constr. .. the rect. DH, DG=the rect. DK, DP:
.: the point H is on the O PKG. Let O be the centre of the O PHG.
Join OG, OH, CH.
and DG meets them.
But since OG=OH, and CD=CH,
the L OHG=the L CHD; :: OH and CH are in one st. line. .. the O PHG touches the given O DHE. Q. E. F,
Notes. (i) Since two circles may be drawn to pass through P, K and to touch AB, it follows that there will be two solutions of the present problem,
(ii) Two more solutions may be obtained by joining PE, and proceeding as before.
The student should examine the nature of the contact between the circles in each case.
26. Describe a circle to pass through a given point, to touch a given straight line, and to have its centre on another given straight line.
27. Describe a circle to pass through a given point, to touch a given circle, and to have its centre on a given straight line.
28. Describe a circle to pass through two given points, and to intercept an arc of given length on a given circle.
29. Describe a circle to touch a given circle and a given straight line at a given point.
30. Describe a circle to touch two given circles and a given straight line.
We gather from the Theory of Loci that the position of an angle, line or figure is capable under suitable conditions of gradual change; and it is usually found that change of position involves a corresponding and gradual change of magnitude.
Under these circumstances we may be required to note if any situations exist at which the magnitude in question, after increasing, begins to decrease ; or after decreasing, to increase : in such situations the magnitude is said to have reached a Maximum or a Minimum value ; for in the former case it is greater, and in the latter case less than in adjacent situations on either side. In the geometry of the circle and straight line we only meet with such cases of continuous change as admit of one transition from an increasing to a decreasing state-or vice versa—so that in all the problems with which we have to deal (where a single circle is involved) there can be only one Maximum and one Minimum—the Maximum being the greatest, and the Minimum being the least value that the variable magnitude is capable of taking.
Thus a variable geometrical magnitude reaches its maximum or minimum value at a turning point, towards which the magnitude may mount or descend from either side : it is natural therefore to expect a maximum or minimum value to occur when, in the course of its change, the magnitude assumes a symmetrical form or position ; and this is usually found to be the case.
This general connection between a symmetrical form or position and a maximum or minimum value is not exact enough to constitute a proof in any particular problem ; but by means of it a situation is suggested, which on further examination may be shewn to give the maximum or minimum value sought for.
For example, suppose it is required to determine the greatest straight line that may be drawn perpendicular to the chord of a segment of a circle and intercepted between the chord and the arc : we immediately anticipate that the greatest perpendicular is that which occupies a symmetrical position in the figure, namely the perpendicular which passes through the middle point of the chord ; and on further examination this may be proved to be the case by means of 1. 19, and 1. 34.
Again we are able to find at what point a geometrical magnitude, varying under certain conditions, assumes its Maximum or Minimum value, if we can discover a construction for drawing the magnitude so that it may have an assigned value: for we may then examine between what limits the assigned value must lie in order that the construction may be possible; and the higher or lower limit will give the Maximum or Minimum sought for.
It was pointed out in the chapter on the Intersection of Loci, (see page 125] that if under certain conditions existing among the data, two solutions of a problem are possible, and under other conditions, no solution exists, there will always be some intermediate condition under which one and only one distinct solution is possible.
Under these circumstances this single or limiting solution will always be found to correspond to the maximum or minimum value of the magnitude to be constructed.
1. For example, suppose it is required to divide a given straight line so that the rectangle contained by the two segments may be a maximum.
We may first attempt to divide the given straight line so that the rectangle contained by its segments may have a given area—that is, be equal to the square on a given straight line.
Let AB be the given straight line, and K the side of the given square,
It is required to divide the st. line AB at a point M, so that
the rect. AM, MB may be equal to the sq. on K. Adopting a construction suggested by II. 14,
describe a semicircle on AB; and at any point X in AB, or AB produced, draw XY perp. to AB, and equal to K.
Through Y draw YZ parł to AB, to meet the arc of the semicircle at P.
Then if the perp. PM is drawn to AB, it may be shewn after the manner of 11. 14, or by 111. 35 that the rect. AM, MB=the sq. on PM
=the sq. on K. So that the rectangle AM, MB increases as K increases.
Now if K is less than the radius CD, then YZ will meet the arc of the semicircle in two points P, P'; and it follows that AB may be divided at two points, so that the rectangle contained by its segments may be equal to the square on K. If K increases, the st. line YZ will recede from AB, and the points of intersection P, P' will continually approach one another; until, when K is equal to the radius CD, the st. line YZ (now in the position Y'Z') will meet the arc in two coincident points, that is, will touch the semicircle at D; and there will be only one solution of the problem.
If K is greater than CD, the straight line YZ will not meet the semicircle, and the problem is impossible.
Hence the greatest length that K may have, in order that the construction may be possible, is the radius CD.
: the rect. AM, MB is a maximum, when it is equal to the square on CD;
that is, when PM coincides with CD, and consequently when M is the middle point of AB.
NOTE. The special feature to be noticed in this problem is that the maximum is found at the transitional point between two solutions and no solution ; that is, when the two solutions coincide and beconie identical.