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angle*, and because in the right angled triangle ADC, 31. 5. BĎ is drawn from the right angle perpendicular to the base, DB is a mean proportional between AB, BC the segments of the base*: therefore between the * Cor. 8. 6. two given straight lines AB, BC, a mean proportional DB is found. Which was to be done.

PROP. XIV. THEOR.

Equal parallelograms, which ave one angle of the one

equal to one angle of the other, have their sides about the equal angles reciprocally proportional : and parallelograms that have one angle of the one equal to one angle of the other, and their sides about the equal angles reciprocally proportional, are equal to one another.

F

E

B

* 7. 5.

* 1.6.

Let AB, BC be equal parallelograms which have the angles at B equal : the sides of the parallelograms AB, BC about the equal angles, shall be reciprocally proportional; that is, DB shall be to BE, as GB to BF.

Let the sides DB, BE be placed in the same straight line; wherefore also FB, BG are in one straight line*; * 14. 1. complete the parallelogram FE: and because the parallelogram AB is equal to BC, and that FE is another parallelogram, AB is to FE, as BC to FE*: but as AB to FE, so is the base DB to BE*, and as BC to FE, so is the base GB to BF; therefore, as DB to BE, so is GB to BF* Wherefore, the sides of the pa- * 11.5. rallelograms AB, BC about their equal angles are reciprocally proportional.

Next, let the sides about the equal angles be reciprocally proportional, viz. as DB to BĘ, so GB to BF; the parallelogram AB shall be equal to the parallelogram BC.

Because, as DB to BE, so is GB to BF; and as DB to BEt, so is the parallelogram AB to the paral- +1.6. lelogram FÊ; and as GB to BF, so is the parallelogram BC to the parallelogram FE; therefore* as AB to FE, so BC to FE: therefore the parallelogram AB is equal* to the parallelogram BC. Therefore equal parallelograms, &c.

* 11.5.

* 9. 5.

Q. E. D.

PROP. XV. THEOR.

Equal triangles which have one angle of the one equal to

one angle of the other, have their sides about the equal angles reciprocally proportional: and triangles which have one angle in the one equal to one angle in the other, and their sides about the equal angles reciprocally proportional, are equal to one another.

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D

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Let ABC, ADE be equal triangles, which have the angle BAC equal to the angle DAE: the sides about the equal angles of the triangles shall be reciprocally proportional; that is, CA shall be to AD, as EA to AB.

Let the triangles be placed so that their sides CA, AD be in one straight line; wherefore also EA and AB are in one straight line*; and join BD. Because the triangle ABC is equal to the

E triangle ADE, and that ABD is another triangle; therefore as the triangle CAB, is to the triangle BAD*, so is the triangle AED to the triangle DAB: but as the triangle CAB to the triangle BÅD, so is the base CA to AD*, and as the triangle EAD to the triangle DAB, so is the base EA to AB*; therefore as CA to AD*, so is EA to AB: wherefore the sides of the triangles ABC, ADE about the equal angles are reciprocally proportional.

Next, let the sides of the triangles ABC, ADE about the equal angles be reciprocally proportional, viz. CA to AD, as EA to AB; the triangle ABC shall be equal to the triangle ADE.

Join BD as before: then, because as CA to AD, so is EA to AB; and as CA to AD, so is the triangle ABC to the triangle BAD*; and as EA to AB, so is the triangle EAD to the triangle BAD*; therefore* as the triangle BAC to the triangle BAD, so is the triangle EAD to the triangle BAD; that is, the triangles BAC, EAD have the same ratio to the triangle BAD: wherefore the triangle ABC is equal* to the triangle ADE. Therefore equal triangles, &c. Q. E. D.

* 1. 6. * 1.6. * 11. 5.

* 9.5.

PROP. XVI. THEOR.

* 11.1.

If four straight lines be proportionals, the rectangle contained by the extremes is equal to the rectangle contained by the means : and if the rectangle contained by the extremes be equal to the rectangle contained by the means, the four straight lines are proportionals.

Let the four straight lines AB, CD, E, F, be proportionals, viz. as AB to CD, so E to F: the rectangle contained by AB, F shall be equal to the rectangle contained by CD, E.

From the points A, C draw * AG, CH at right angles to AB, CD: and maket AG equal to F, and CH + 3. 1. equal to E; and compleie + the parallelograms BG, + 31. 1. DH. Because, as AB to CD, so is E to F; and that E is equal to CH, and F to AG; AB is * to CD as * 7.5. CH to AG: therefore the sides of the parallelograms BG, DH, about the equal angles are reciprocally proportional; but parallelograms which have their sides about equal angles reciprocally proportional, are equal

* 14. 6. to one another *; therefore the parallelogram BG is equal to the parallelogram DH: but the parallelogram BG is contained by the straight lines AB, F, because AG is equal to F; and the parallelogram DH is contained by E CD and E, because CH is equal to

с. E; therefore the rectangle contained by the straight lines AB, F is equal to that which is contained by CD and E.

And if the rectangle contained by the straight lines AB, F be equal to that which is contained by CD, E; these four lines shall be proportional, viz. AB shall be to CD, as E to F.

The same construction being made, because the rectangle contained by the straight lines AB, F is equal to that which is contained by CD, E, and that the rectangle BG is contained by AB, F, because AG is equal to F: and the rectangle DH by CI), E, because CH is equal to E; therefore the parallelogram BG is equal + to the parallelogram DH; and they are +1 Ax. equiangular : but the sides about the equal angles of equal parallelograms are reciprocally proportional * :

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D

* 14.6.

+ 7.5.

wherefore, as AB to CD, so is CH to AD: but CH is
equal to E, and

AG to F; therefore as AB is to CD +,
so is E to F. Wherefore, if four, &c. Q. E. D.

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If three straight lines be proportionals, the rectangle con

tained by the extremes is equal to the square of the
mean : and if the rectangle contained by the extremes
be equal to the square of the mean, the three straight
lines are proportionals.

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*7.5.

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B

* 16.6.

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Let the three straight lines A, B, C be proportionals,
viz. as A to B, so B to C: the rectangle contained by
A, C shall be equal to the square of B.

Take D equal to B: and because as A to B, so B to C,
and that B is equal to D; A is * to B, as D to C: but
if four straight lines be proportionals the rectangle con-
tained by the extremes is equal to
that which is contained by the
means *; therefore the rectangle D
contained by A, C is equal to that
contained by B, D: but the rect-

Ic
angle contained by B, D is the
square of B, because B is equal
to D; therefore the rectangle contained by A, C is equal
square

of B.
And if the rectangle contained by A, C be equal to
the

square of B; A shall be to B, as B to C.
The same construction being made, because the
rectangle contained by A, C is equal to the square of
B, and the square of B is equal to the rectangle con-
tained by B, D, because B is equal to D; therefore
the rectangle contained by A, C is equal to that con-
tained by B, D: but if the rectangle contained by the
extremes be equal to that contained by the means, the
four straight lines are proportionals * : therefore A is
to B, as D to C: but B is equal to D; wherefore, as
A to B, so B to C. Therefore if three straight lines,
&c.

Q. E. D.

to the

* 16.6.

PROP. XVIII. PROB.

Upon a given straight line to describe a rectilineal figure See N.

similar, and similarly situateil, to a given rectilineal figure.

32. 1. & 3 Ax.

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. 23. 1.

Let AB be the given straight line, and CDEF the given rectilineal figure of four sides ; it is required upon the given straight line AB to describe a rectilineal figure similar, and similarly situated, to CDEF.

Join DF, and at the points A, B in the straight line AB make* the angle BAG equal to the angle at C, . 23. 1. and the angle ABG equal to the angle CDF; therefore the remaining angle CFD is equal to the remaining angle AGB *: therefore the triangle FCD is equiangular to

G the triangle GAB: again, at the points, G, B in the straight line

K GB, make* the angle BGH equal в с to the angle DFE, and the angle GBH equal to FDE; therefore the remaining angle FED is equal to the remaining angle GHB, and the triangle FDE equiangular to the triangle GBH: then, because the angle AGB is equal to the angle CFD, and BGH to DFE, the whole angle AGH is equal to the + ? Ax. whole CFE; for the same reason, the angle ABH is equal to the angle CDE: also the angle at A is equal + + Constr. to the angle at C, and the angle GHB to FED: therefore the rectilineal figure ABHG is equiangular to CDEF: likewise these figures have their sides about the equal angles proportionals: because the triangles GAB, FCD being equiangular, BA is * to AG, as DC * 4.6. to CF; and because AG is to GB, as CF to FD; and as GB to GH, so, by reason of the equiangular triangles BGH, DFE, is FD to FE; therefore, ex æquali *, AG is to GH, as CF to FE: in the same manner it may be proved that AB is to BH, as CD to DE: and GH is to HB*, as FE to ED. Wherefore, because the tilineal figures ABHG, CDEF are equiangular, and have their sides about the equal angles proportionals, they are similar * to one another.

Next, let it be required to describe upon a given straight line AB, a rectilineal figure similar, and simi

22. 5.

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* 4. 6.

* 1 Def. 6.

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