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Join DA and BD to divide the given figure into triangles, and produce | AB indefinitely. D M
N Through. E draw EH parallel to DA, and through .C, CF parallel to DB : join DH and DF;
E then the triangle DHF is equal in area to the
С given figure ABCDE.
Next, through. D draw DN parallel to HF; bisect HF, a side of the triangle DHF, in L; at. L raise the perpendicular LM, and through H A LB . F draw FN a parallel to L M; the fig. LMNF is a rectangle of the same altitude as the triangle, and on half its base,—and is therefore equal in area to the triangle DHF.
3. In a similar way a crooked boundary, ABCDE, between two fields, M and N, may be made straight without changing the relative size of the fields. Draw AC the subtend to angle B, and
B boundary ; and thus the crooked boundaries A B, BC, CD, and DE, will be changed into one straight boundary without affecting the size of the fields : the shape will be altered, but the areas
A F of fields M and N remain as at first.
PROP. 46. -PROB.
To describe a square on a given straight line.
Sol.-P. 11. To draw a st. line at right angles to a given st. line from a
given point in the same.
are equal and parallel, and the diagonals join opposite angles. DEM.-P. 34. The opposite sides and angles of parallelograms are equal.
Ax. 1. Things equal to the same, are equal to each other.
angles equal to each other; and the ext. angle equal to the int. and
two rt. angles. Ax. 3. If equals be taken from equals, the remainders are equal. Def. 30. Of four-sided figures a square is that which has all its sides
equal, and all its angles rt. angles.
Exp. 1 | Datum.
Given the st. line AB;
Cons. 1 by P. 11. From . A draw A C at rt. angles
to AB; 2 P. 3. make AD
JB 3 P. 31. through. D draw DE || AB;
and through . B, BE || AD;
and the square required.
AD; 2 | Ax. 1.
AD DE EB, 3 D. 1 & 2. and .. the OABD E is equilateral. 4 C. 3. Also, : AD meets the parallels AB and DE, 5 P. 29. :, the int. LS BAD, ADE two rt. angles : 6 C. 1.
but the BAD is a rt. angle ; 7 Ax. 3. :. _ ADE is also a rt. angle. 8 P. 34. Now the opposite angles of os are equal ; 9 D. 7 & 6. :. _ ABE opposite to Z ADE is a rt. angle,
and 1 BED opposite to 2 BAD is a rt. angle;
it is also equilateral ;
Q.E. F. Cor. 1.—The squares on equal st. lines are equal; and if the squares are equal, the st. lines are equal."
2. Every parallelogram having one right angle, has all its angles rt. angles.
Sch.—Given the diagonal AB to construct a square.
E 4 Sol. then ABCD is the square required. DEM. 1 byC.2,&P.6. ::the zs CAB and C B A are equal,
side AC = side CB;
Again, the 2s CAB, CB A being together one rt, engle, 5 P. 32, the angle C is a rt. angle ;
Dem. 6 P.46, Cor.2. But in a , as ACBD, when one angle is a rt. angle, all
the angles are rt. angles ; 7 C. 3, Concl. :. also ACBD has its angles rt. angles ; 8 Concl. And therefore, the figure being equilateral and rectan
gular, ACBD is the square required. USE AND APP.—The Geometrical Square is an instrument by means of which, and of the property of similar triangles that the sides about the equal angles are proportional, the height of an inaccessible object can be ascertained, provided a measurement to the perpendicular from the object can be made. The edges of the square are each divided into 100 equal parts, and from one corner a plummet is suspended ; when the object is seen along one edge of the instrument, the plummet cuts another edge, and forms a triangle similar to the triangle formed by lines representing the perpendicular from the object, a parallel to the horizontal line at its base, and the hypotenuse, or distance from the point of observation to the object itself.
In the adjoining figures, | A B represents
C the horizon ; p D a parallel to the horizon ; DB the height of the instrument; CD the height of the object C above the parallel to the horizon ; sp the edge along which the object is to be seen ; sr, rn, and pn graduated edges each of 100 parts; and p the point of suspension for the plummet. From the place of observativn measure
D the distance p D, and the height of the instrument DB; direct the edge sp towards the object C, and note the number of parts in sr or in rn. 1°. When the plummet cuts А
IB 8r in o, the triangle pso is similar to the
С triangle CDp; and we have the proportion 80:88:: PD: CD; whence CD = $p.pD and CB = CD + D B. 2o. When the plummet cuts rn in o, the triangles on p and
D CDp are similar ; and we have the proportiou pn : no :: PD:DC; whence CD = no.pD ; and CB = CD + DB.
B For example, let p D = 60 ft. ; 80 = 50 eq. pts. ; and DB = 6 ft. ; required
; CB. Here 50 : 100 :: 6n : 120 = CD, and 120 + 6 = 126 ft. = CB.-Sec Tate's Geometry, pp. 49-51.
Prop. 47.—THEOR.—(Most Important.) In any right-angled triangle, the square which is described upon the side subtending, or opposite to, the right angle, is equal to the squares described upon the sides containing the right angle. Cons.—P. 46. To describe a square on a given straight line.
P. 31. Through a point to draw a st. line parallel to a given st. line.
Pst. 1. Any two points may be joined by a st. line. Dem.-Def. 30. A square has all its sides equal, and its angles rt. angles. P. 14. If at a point in a st. line two other st.. lines on the opposite
si les of it make the adjacent anzles together equal to two rt. angles,
these two st. lines shall be in one and the same st. line.
the triangles are in every other respect equal.
between the same parallels, the parallelogram is double of the
Ax. 6. Things double of the same, are equal to each other. EXP. 1 Нур.
Let ABC be a A
and BAC a rt.
then the square
Cons.1 by P. 46.
Draw on BC a
AL || BD or to CE;
2 P. 31.
3 Pst. 1.
DEM. 1 by H&Def.30 : 28 BAC and B AG are each a rt. angle ; 2 P. 14. :: the lines AC, AG on opp. sides of A B make
the adj. LS two rt. angles, and :. CA is in the same st. line with AG.
3 H. Def. 30 Also AB is on the same st. line with AH.
DEM. 41C.1, Def. 301 :: the Ls DBC
and FBA are
the L FBA:
K equals add the
whole L FBC: 8 C.1,&Def.30 Hence, .. AB = FB, B D = BC,
L E and – DBA = ¿FBC; 9 P. 4. .:. base A D = base FC, and A ABD= AFBC. 10 C. 2.
Now the BL and the A ABD are both
on the same base BD, and between the same
parallels BD and AL; 11 P. 41. .. the BL is double of the A ABD. 12 C. 2. Also the square GB and the A FBC are
both on the same base FB, and between the
same parallels FB and G C; 13 P. 41. .. the
or square. GB, is double of the
GB. 16 C. 3. Also, after joining A E, BK, the CL =
the square HC: 17 Ax. 2.
:. the whole square BDEC = the two squares
GB and HC;
BG on side BA, and CH on side CA;
on sides BA and CA.
Q.E.D. CoR. I.--Hence, if the sides of a rt. angled triangle be given in numbers, its hypotenuse may be found : for, let the squares of the sides be added together, and the square root of their sum will be the hypotenuse. Suppose AB the base, AC the perpendicular, BC the hypotenuse; the formula for B C is,
the two squares