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Book II.

PROP. VI. THEOR.

a 46. 1.

If a straight line be bisected, and produced to any point: the rectangle contained by the whole line thus produced, and the part of it produced, together with the square of half the line bisected, is equal to the square of the straight line which is made up of the half and the part produced.

Let the straight line AB be bisected in C, and produced to the point D; the rectangle AD, DB, together with the square of CB, is equal to the square of CD.

Upon CD describea the square CEFD, join DE, and 131. 1. through B drawb BHG parallel to CE or DF, and through

H draw KLM parallel to AD or EF, and also through
A draw AK parallel to CL
or DM; and because AC

А.

B is equal to CB, the rectan• 36. 1. gle AL is equal to CH;

L

H • 43. 1. but CH is equals to KFR

therefore also AL is equal
to HF: To each of these
add CM; therefore the
whole AM is equal to the

E goomon CMG: And AM Cor. 4.2 is the rectangle contained by AD, DB, for DM is equale

to DB: Therefore the gnomon CMG is equal to the rectangle AD, DB: Add to each of these LG, which is equal to the square of CB; therefore the rectangle AD, DB, together with the square of CB, is equal to the gnomon CMG, and the figure LG; But the gnomon CMG and LG make up the whole figure CEFD, which is the square of CD; therefore the rectangle AD, DB, together with the square of CB, is equal to the square of CD. Wherefore, if a straight line, &c. Q. E. D.

PROP. VII. THEOR. If a straight line be divided into any two parts, the squares of the whole line, and of one of the parts, are equal to twice the rectangle contained by the whole and that part, together with the square of the

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other part.

Let the straight line AB be divided into any two parts in

the point C; the squares of AB, BC are equal to twice the Book II. rectangle AB, BC, together with the square of AC.

Upon AB describe the square ADEB, and construct the-46. 1. figure as in the preceding propositions; and because AG is equal to GE, add to each of them CK; the whole AK is 143. , therefore equal to the whole CE; therefore AK, CE, are double of A С AK: But AK, CE, are the gnomon AKF, together with the square CK; therefore the gnomon AKF, toge-H

K ther with the square CK, is double of AK: But twice the rectangle AB, BC is double of AK, for BK is equal to BC: Therefore the gno

¢ Cor. 4. 2 mon AKF, together with the square D

E CK, is equal to twice the rectangle AB, BC: To each of these equals add HF, which is equal to the square of AC; therefore the gnomon AKF, together with the squares CK, HF, is equal to twice the rectangle AB, BC, and the square of AC : but the gnomon AKF, together with the squares CK, HF, make up the whole figure ADEB and CK, which are the squares of AB and BC: therefore the squares of AB and BC are equal to twice the rectangle AB, BC, together with the square of AC. Wherefore, if a straight line, &c. Q. E. D.

PROP. VIII. THEOR.

If a straight line be divided into any two parts, four times the rectangle contained by the whole line, and one of the parts, together with the square of the other part, is equal to the square of the straight line, which is made up of the whole and that part.

Let the straight line AB be divided into any two parts in the point C; four times the rectangle AB, BC, together with the square of AC, is equal to the square of the straight line made up of AB and BC together.

Produce AB to D, so that BD be equal to CB, and upon AD describe the square AEFD; and construct two figures such as in the preceding. Because CB is equal to BD, and that CB is equala to GK, and BD to KN; therefore GK is ^ 34. 1.

E

Book IỊ. equal to KN: For the same reason, PR is equal to RO; and n because CB is equal to BD, and GK to KN, the rectangle b36. 1. CK is equalb to BN, and GR to RN; but CK is equal to € 43. 1. RN, because they are the complements of the parallelogram

CO; therefore also BN is equal to GR; and the four rectangles BN,CK, GR, RN are therefore equal to one another, and so are quadruple of one of them CK: Again, because CB

is equal to BD, and that BD is *Cor. 4. 2. equal d to BK, that is, to CG,

and CB equal to GK, that d is, to
GP; therefore CG is equal to

с в
GP: And because CG is equal to

GK
GP, and PR to RO, the rectangle

N
AG is equal to MP, and PL to

PI

X € 43. 1. RF: But MP is equal e to PL,

R

0 because they are the complements of the parallelogram ML; wherefore AG is equal also to RF: Therefore the four rectangles AG, MP, PL, RF, are equal to one

E H another and so are quadruple of one of them AG. And it was demonstrated that the four CK, BN, GR, and RN are quadruple of CK. Therefore the eight rectangles which contain the gnomon AOH, are quadruple of AK; and because AK is the rectangle contained by AB, BC, for BK is equal to BC, four times the rectangle AB, BC is quadruple of AK: But the gnomon AOH was demonstrated to be quadruple of AK; therefore four times

the rectangle AB, BC, is equal to the gnomon AOH. To * Cor. 4. 2. each of these add XH, which is equalf to the square of AC:

Therefore four times the rectangle AB, BC together with the square of AC, is equal to the gnomon AOH and the square XH: But the gnomon AOH and XH make up the figure AEFD, which is the square of AD: Therefore four times the rectangle AB, BC, together with the square of AC, is equal to the square of AD, that is, of AB and BC added together in one straight line. Wherefore, if a straight line, &c. Q. E. D.

Book II.

PROP. IX. THEOR.

If a straight line be divided into two equal, and also into two unequal parts; the squares of the two unequal parts are together double of the square of half the line, and of the square of the line between the points of section.

Let the straight line AB be divided at the point C into two equal, and at D into two unequal parts : The squares of AD, DB are together double of the squares of AC, CD.

From the point C drawa CE at right angles to AB, and " 11. 1. make it equal to AC or CB, and join EA, EB; through D . draw bDF parallel to CE, and through F draw FG parallel b31. 1. to AB; and join AF: Then, because AC is equal to CE, the angle EAC is equal to the angle AEC; and because the c5. 1. angle ACE is a right angle, the two others AEC, EAC together make one right angled; and they are equal to one • 32. 1. another; each of them therefore is half of a right angle.

E For the same reason each of the angles CEB, EBC is half a

G

F right angle; and therefore the whole AEB is a right angle: And because the angle GEF is

C С half a right angle, and EGF a

D B right angle, for it is equale to the interior and opposite an- « 29. 1. gle ECB, the remaining angle EFG is half a right angle ; therefore the angle GEF is equal to the angle EFG, and the side EG equalf to the side GF: Again, because the f6. 1. angle at B is half a right angle, and FDB a right angle, for it is equale to the interior and opposite angle ECB, the remaining angle BFD is half a right angle; therefore the angle at B is equal to the angle BFD, and the side DF tof the side DB :. And because AC is equal to CE, the square of AC is equal to the square of CE; therefore the squares of AC, CE, are double of the square of AC: But the square of EA is equals to the squares of AC, 547. I. CE, because ACE is a right angle; therefore the square of EA is double of the square of AC: Again, because EG is equal to GF, the square of EG is equal to the square of GF; therefore the squares of EG, GF are double of

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Book II. the square of GF; but the square of EF is equal to the

squares of EG, GF; therefore the square of EF is double b 34. 1. of the square GF; and GF is equalh to CD; therefore the

square of EF is double of the square of CD: But the square of AE is likewise double of the square of AC;

therefore the squares of AE, EF are double of the squares i 47. 1. of AC, CD : And the square of AF is equali to the squares

of AE, EF, because AEF is a right angle; therefore the square of AF is double of the squares of AC, CD: But the squares of AD, DF, are equal to the square of AF, because the angle ADF is a right angle; therefore the squares of AD, DF are double of the squares of AC, CD: And DF is equal to DB ; therefore the squares of AD, DB are double of the squares of AC, CD. If therefore a straight line, &c. Q. E. D.

PROP. X. THEOR.

If a straight line be bisected, and produced to any point, the square of the whole line thus produced, and the square of the part of it produced, are together double of the square of half the line bisected, and of the square of the line made

up

of the half and the part produced.

Let the straight line AB be bisected in C and produced to the point D; the squares of AD, DB are double of the

squares of AC, CD. 'a 11.1. From the point C draw a CE at right angles to AB:

And make it equal to AC or CB, and join AE, EB; • 31. 1. through E draw bEF parallel to AB, and through D draw

DF parallel to CE: And because the straight line EF

meets the parallels EC, FD, the angles CEF, EFD are € 99. 1. equal to two right angles; and therefore the angles BEF,

EFD are less than two right angles; but straight lines

which with another straight line make the interior angles * 12 Ax. upon the same side less than two right angles, do meerd if

produced far enough: Therefore EB, FD shall meet, if pro

duced towards B, D: Let them meet in G, and join AG: * 5. 1. Then, because AC is equal to CE, the angle CEA is equale

to the angle EAC; and the angle ACE is a right angle; € 32. 1. therefore each of the angles CEA, EAC is half a right angle."

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