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PROP. XXI. THEOR.
RECTILINEAL figures which are similar to the same rectilineal figure, are also similar to one another.
Let each of the rectilineal figures, A, B be similar to the rectilineal figure C: the figure A is similar to the figure B.
Because Å is similar to C, they are equiangular, and also have their sides about the equal angles proportionals 2. Again, & 1. Det. 6: because B is similar to C, they are equiangular, and have their sides about the equal angles propor
B tionals a ; . therefore the figures A, B are each of them equiangular to C, and have the sides about the equal angles of each of them and of C proportionals. Wherefore the rectilineal figures A and B are equiangular", and b 1. Ax. 1. have their sides about the equal angles proportionals c. There-c 11. 5: fore A is similar to B. Q. E. D.
PROP. XXII. THEOR.
IF four straight lines be proportionals, the similar rectilineal figures similarly described upon them shall also be proportionals; and if the similar rectilineal figures similarly described upon four straight lines be proportionals, those straight lines shall be proportionals.
Let the four straight lines AB, CD, EF, GH be proportionals, viz. AB to CD, as EF to GH, and upon AB,CD let the similar rectilineal figures KAB, LCD be similarly described ; and upon EF, GH the similar rectilineal figures MF, NH in like manner: the rectilineal figure KAB is to LCD, as MF to NH.
To AB, CD take a third proportional a X; and to EF, GH 2 11.6. a third proportional 0 : and because AB is to CD, as EF to b 11. 5. GH, and that CD is b to X, as GH to 0; wherefore, er equali, c 22. 5. as AB to X, so EF to 0: but aş AB to X, so is d the rectilineal a ? Cor. 20.
Book VI. KAB to the rectilineal LCI), and as EF to O, so is d the rection lineal MF to the rectilineal NH; therefore, as KAB to LCD,
so bis MF to NH. 20. 6.
And if the rectilineal KAB be to LCD, as MF to NH; the b 11. 5. straight line AB is tò CD, as EF, to GH. e 12. 6.
Make e as AB to CD, so EF to PR, and upon PR describe! f 18. 6. the rectilineal figure SR simitar and similarly situated to either
of the figures MF, NH: then, because as AB to CD, soris
8 9. 5.
PROP. XXIII. THEOR.
EQUÍANGULAR Parallelograms have to one another the ratio which is compounded of the ratios of their sides.
Let AC, CF be equiangular parallelograms, having the angle BCD equal to the angle ECG: the ratio of the parallelogram AC to the parallelogram CF is the same with the ratio which is compounded of the ratios of their sides.
Let BC, CG, be placed in a straight line; therefore DC and Book VI. CF are also in a straight line a ; and complete the parallelogram DG; and, taking any straight line K, makebas BC to CG, a 14, 1. so K to L, and as DC to CE, so makeb L to M: therefore b 12. 6. the ratios of K to L, and L to M, are the same with the ratios of the sides, viz. of BC to CG, and DC to ÇE. But the ra-:tio of K to M is that which is said to be compoundedc of the c A. def. 5. ratios of K to 1., and I to M; wherefore also K has to M the ratio compounded of the ratios of the
e 11. 5. rallelogram AC to the parallelogram B
C CH: again, because as DC to CE, so is the parallelogram CH to the parallelogram CF; but as DC to CE, so is L to M; wherefore Lise to M, as the parallelogram CH to the parallelogram CF: therefore, since it has been K L M E proved, that as K to L, so is the parallelogram Ac to the pa. rallelogram CH; and as L to M, so the parallelogram CH to the parallelogram CF; ex æqualis, K is to M, as the paral-f 22. 5. lelogram AC to the parallelogram CF: but K has to M the ratio which is compounded of the ratios of the sides; therefore also the parallelogram AC has to the parallelogram CF the ratio which is compounded of the ratios of the sides. Wherefore, equiangular parallelograms, &c. Q. E. D.
PROP. XXIV. THEOR.
THE parallelograms about the diameter of any parallelogram, are similar to the whole, and to one another,
Let ABCD be a parallelogram, of which the diameter is AC; and EG, HK the parallelograms about the diameter: the parallelograms EG, HK are similar both to the whole parallelogram ABCD, and to one another,
Because DC, GF are parallels, the angle ADC is equal a to the angle AGF: for the same reason, because BC, EF are pa
Book VI. ralleis, the, angle ABC is equal to the angle AEF: and each
of the angles BCD, EFG is equal to the opposite angle DAB, b 34. 1. and therefore are equal to one another; wherefore the paral
lelograms ABCD, AEFG are equiangular : and because the angle ABC is equal to the angle AEF, and the angle BAC
common to the two triangles BAC, EAF, they are equiangu• 4. 6. lar/to one another; therefore cas-AB A
В. to BC, so is AE to EF: and because the opposite sides of parallelograms
F d. 5. are equal to one anotherb, AB igd to G
AD, as AE to AG; and DC to CB,
С ¢ 1. def. 6. als'; and they are tlierefore similar to one another e: for the
same reason, the parallelogram ABCD is similar to the parallelogram FHCK. Wherefore each of the parallelograms GE, KH is similar to DB: but rectilinçal figures' which are
similar to the same rectilineal figure, are also similar to one £21. 6. anotherf; therefore the parallelogram GE is similar to KH.
Wherefore, the parallelograms, &c. Q. E. D.
PROP. XXV. PROB.
See Note. TO describe a rectilineal figure which shall be si
milar to one, and equal to another given rectilineal igure.
Let ABC be the given rectilineal figure, to which the figure to be described is required to be similar, and D that to which it must be equal. It is required to describe a rectilineal figure
similar to ABC, and equal to D. a Cor. 45. 1. Upon the straight line BC describe the parallelogram BE
equal to the figure ABC; also upon CE describe a the parallelogram CM equal to D, and having the angle FCE equal to the angle CBI:: therefore BC and CF are in a straight
lipéb, as also LE and EM: between BC and CF finde a mean b 14. 1.
proportional GH, and upon GH described the rectilineal figure c 13. 6. d 18. 6.
KGH similar and similarly situated to the figúre ABC: and e 2. Cor. 20. because BC is to GH as GH to CF, and if three straight
6. Lines be proportionals, as the first is to the third, so ise the
$ 29, 1.
figure upon the first to the similar and similarly described figure Book VI.
equal to the parallelogram BE; therefore the rectilineal
PROP. XXVI. THEOR,
IF two similar parallelograms have a common an. gle, and be similarly situated; they are about the same diameter.
Lot the parallelograms ABCD, AEFG be similar and simi-
For, if not, let, if possible, the A G
H line from AF the diameter of the
so ige the