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IF from the vertex of a triangle given in species, a straight line be drawn to the base in a given angle; it shall have a given ratio to the base.

From the vertex A of the triangle ABC which is given in species, let AD be drawn to the base BC in a given angle ADB; the ratio of AD to BC is given. Because the triangle ABC is given in spe

А cies, the angle ABD is given, and the angle ADB is given, therefore the triangle ABD is given a in species; wherefore the ratio of AÐ

a 43. dat. to AB is given. And the ratio of AB to BC

B

D is given ; and therefore b the ratio of AD to

Ch 9. dat. BC is given.

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RECTILINEAL figures given in species, are divided into triangles which are given in species.

Let the rectiligeal figure ABCDE be given in species : ABCDE may be divided into triangles given in species.

Join BE, BD; and because ABCDE is given in species, the angle BAE is given a, and the ratio of BA

A

a 3. def. to AE is given a ; wherefore the triangle BAE is given in species b, and the angle

b 44, dat. AEB is therefore given a. But the whole angle AED is given, and therefore the re-B

E maining angle BED is given, and the ratio of AE to EB is given, as also the ratio of AE to ED; therefore the ratio of BE to с D ED is given c. And the angle BED is given, wherefore the tri-c 9. dat. angle BED is given b in species. In the same manner, the triangle BDC is given in species: therefore rectilineal figures which are given in species are divided into triangles given in species,

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IF two triangles given in species be described upon the same straight line; they shall have a given ratio to one another.

Let the triangle ABC, ABD given in species be described upon the same straight line AB; the ratio of the triangle ABC to the triangle ABD is given.

Through the point C, draw CE parallel to AB, and let i: meet DA produced in E, and join BE. Because the triangle ABC is given in species, the angle BAC, that is, the angle ACE, is given ; and because the triangle ABD is given in species, the angle DAB, that is, the angle AEC E

с is given. Therefore the

L

н triangle ACE is given in species; wherefore the ratio of EA to AC

BT

G A a 3. def. is given a, and the ra

tio of CA to AB is
given, as also the ratio

D of BA to AD; thereb 9. dat. fore the ratio of b EA to AD is given, and the triangle ACB is c 37. 1. equal c to the triangle AEB, and as the triangle AEB, or ACB, d 1. 6.

is to the triangle ADB, so is d the straight line EA to AD. But the ratio of EA to AD is given, therefore the ratio of the triangle ACB to the triangle ADB is given.

PROBLEM.

© 23. 1.

To find the ratio of two triangles ABC, ABD given in species, and which are described upon the same straight line AB.

Take a straight line FG given in position and magnitude. and because the angles of the triangles ABC, ABD are given, at the points F, G of the straight line FG, make the angles GFH, GFK e equal to the angles BAC, BAD; and the angles FGH, FGK equal to the angles ABC, ABD, each to each. Therefore the triangles ABC, ABD are equiangular to the triangles FGH, FGK, each to each. Through the point H draw HL parallel to FG meeting KF produced in L. And because the angles BAC, BAD are equal to the angles GFH, GFK, each to each ; therefore the angles ACE, AEC are equal to FHL FLH, each to each, and the triangle AEC equiangular to the triangle FLII. Therefore as EA to AC, so is LF to FH; and

as CA to AB, so HF to FG ; and as BA to AD, so is GF to FK; wherefore, ex æquali, as EA to AD, so is LF to FK. But, as was shown, the triangle ABC is to the triangle ABD, as the straight line EA to AD, that is, as LF to FK. The ratio therefore of LF to FK has been found, which is the same with the ratio of the triangle ABC to the triangle ABD.

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IF two rectilineal figures given in species be de. See Note. scribed upon the same straight line; they shall have a given ratio to one another.

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Let any two rectilineal figures ABCDE, ABFG which are given in species, be described upon the same straight line AB; the ratio of them to one another is given.

Join AC, AD, AF; each of the triangles AED, ADC, ACB, AGF, ABF is given a in species. And because the triangles a 51. dat. ADE, ADC given in species are de

D scribed upon the same straight line AD, the ratio of EAD to DAC is E

С

b 52. dat. givenb; and, by composition, the ratio of EACD to DAC is givenc.

c 7. dat.

B And the ratio of DAC to CAB is A given b, because they are described upon the same straight line AC;

G

F therefore the ratio of EACD to ACB

K L MN is givend; and by composition, the

d 9. dat.

H--1O ratio of ABCDE to ABC is given. In the same manner, the ratio of ABFG to ABF is given. But the ratio of the triangle ABC to the triangle ABF is given ; wherefore b, because the ratio of ABCDE to ABC is given, as also the ratio of ABC to ABF, and the ratio ABF to ABFG; the ratio of the rectilineal ABCDE to the rectilineal ABFG is givend.

PROBLEM.

To find the ratio of two rectilineal figures given in species, and described upon the same straight line.

Let ABCDE, ABFG be two rectilineal figures given in species, and described upon the same straight line AB, and join AC, AD, AF. Take a straight line HK given in position and magnitude, and by the 52d dat. find the ratio of the triangle ADE to the triangle ADC, and make the ratio of HK

to KL the same with it. Find also the ratio of the triangle ACD
to the triangle ACB. And make the ratio of KL to LM the
same. Also, find the ratio of the triangle ABC to the triangle
ABF, and make the ratio of LM to MN the same. And lastly,
find the ratio of the triangle AFB to the triangle AFG, and
make the ratio of MN to NO the

D
same. Then the ratio of ABCDE
to ABFG is the same with the ra E

C
tio of HM to MO.
Because the triangle EAD is to

А

B the triangle DAC, as the straight line HK to KL; and as the triangle DAC to CAB, so is the straight

F line KL to LM; therefore, by using K L MN composition as often as the number

H-1-1-1-1-0 of triangles requires, the rectilineal ABCDE is to the triangle ABC, as the straight line HM to ML. In like manner, because the triangle GAF is to FAB, as ON to NM, by composition, the rectilineal ABFG is to the triangle ABF, as MO to NM; and, by inversion, as ABF to ABFG, SO is NM to MO. And the triangle ABC is to ABF, as LM to MN. Wherefore, because as ABCDE to ABC, so is HM to ML; and as ABC to ABF, so is LM to MN; and as ABF to ABFG, so is MN to MG ; ex æquali, as the rectilineal ABCDE to ABFG, so is the straight line HM to MO.

G

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IF two straight lines have a given ratio to one another; the similar rectilineal figures described upon them similarly, shall have a given ratio to one another.

Let the straight lines AB, CD have a given ratio to one another, and let the similar and similarly placed rectilineal figures E, F be described upon them; the ratio of E to F is given.

To AB, CD, let G be a third pro-
portional : therefore as AB to CD,

G
so is CD to G. And the ratio of AB
to CD is given, wherefore the ratio

F
of CD to G is given ; and conse-

A в с D quently the ratio of AB to G is also

H Н

K L a 9. dat. given a. But as AB to G, so is the b 2. Cor. figure E to the figure b F. There20. 6. fore the ratio of E to F is given.

PROBLEM.

To find the ratio of two similar rectilineal figures, E, F, similarly described upon straight lines AB, CD which have a given ratio to one another: let G be a third proportional to AB, CD.

Take a straight line H given in magnitude; and because the ratio of AB to CD is given, make the ratio of H to K the same with it; and because H is given, K is given. As H is to K, so make K to L; then the ratio of E to F is the same with the ratio of H to L: for AB is to CD, as H to K, wherefore CD is to G, as K to L; and, ex æquali, as AB to G, so is H to L: but

b 2 Cor. the figure E is tob the figure F, as AB to G, that is, as H to L.

20. 6. PROP. LV.

51. IF two straight lines have a given ratio to one another; the rectilineal figures given in species described upon them, shall have to one another a given ratio.

Let AB, CD be two straight lines which have a given ratio to one another ; the rectilineal figures E, F given in species and described upon them, have a given ratio to one another.

Upon the straight line AB, describe the figure AG similar and similarly placed to the figure F; and because F is given in species, AG is also given in species: therefore, since the figures E, AG which are given in spe. A

E

B C D cies, are described upon the same straight line AB, the ratio of E

F

G to AG is given a, and because the

a 53. dat, ratio of AB to CD is given, and HKupon them are described the similar and similarly placed rectilineal figures AG, F, the ratio of AG to F is given b: and the ratio of AG to E is given ; there- b 54. dat. fore the ratio of E to F is givenc.

c 9. dat. PROBLEM. To find the ratio of two rectilineal figures E, F given in species and described upon the straight lines AB, CD which have a given ratio to one another.

Take a straight line H given in magnitude ; and because the rectilineal figures E, AG given in species are described upon the same straight line AB, find their ratio by the 53d dat. and make the ratio of H to K the same ; K is therefore given : and because the similar rectilineal figures AG, F are described

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