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Book II.

PROP. VI. THEOR.

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. a 46. 1. Upon CD describe a the square CEFD, join DE, and through b 31. 1. B draw b BHG parallel to CE or DF, and through H draw KLM

parallel to ADor EF, and also through A draw AK parallel to CL
or DM: and because AC is

А
C

B D equal to CB, the rectangle c 36. 1. AL is equal. c to CH; but

L

H d 43. 1. CH is equal d to HF; there

K

M fore also AL is equal to HF: to each of these add CM; therefore the whole AM is equal to the gnomon CMG: and AM is the rectangle contained by AD, DB,

E

G F e Cor.4.2. for DM is equal é 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 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 rect- Book II. angle AB, BC, together with the square of AC.

Upon AB describe a the square ADEB, and construct the a 46. 1. figure as in the preceding propositions : and because AG is equal b to GE, add to each of them CK; the whole AK is b 43. 1. therefore equal to the whole CE; therefore AK, CE are double of A

с B AK: but AK, CE are the gnomon AKF together with the square CK; therefore the gnomon AKF, toge

G ther with the square CK, is double н!

K of AK: but twice the rectangle AB, BC is double of AK, for BK is equal cto BC : therefore the gnomon AKF, together with the square CK, is equal to twice the rectangle AB, BC: to D

F E 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 10 twice the rectangle AB, BC, together with the square of AC. Wherefore, if a straight line, &c. Q. E. D,

c eor. 4.2

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 equal a to GK, and BD to KN ; therefore GK is a 34. 1.

H

Book II. equal to KN; for the same reason, PR is equal to RO; and

because CB is equal to BD, and GK to KN, the rectangle b 36. 1. CK is equal to BN, and GR to RN: but CK is equal e to c 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 d Cor.4.2. equal d to BK, that is, to CG;

CB and CB equal to GK, that d is, to A

D
GP; therefore CG is equal to

G K
GP: and because CG is equal to M м

N
GP, and PR to RO, the rectangle

P. R
AG is equal to MP, and PL to X

0 e 43. 1. RF: but MP is equal e to PL,

because they are the complements
of the parallelogram ML; where-
fore AG is equal also to RF: E

F therefore the four rectangles

H L AG, MP, PL, RF are equal to one another, and so are quadruple of one of them AG. And it was demonstrated, that the four CK, BN, GR, and RN are quad. ruple 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 each of these add XH, which is equal d 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 draw a CE at right angles to AB, and a 11. 1. make it equal to AC or CB, and join EA, EB; through D draw

DF parallel to CE, and through F draw FG parallel to AB; b 31. 1. and join AF: then, because AC is equal to CE, the angle EAC is equal c to the angle AEC; and because the angle c 5. 1. ACE is a right angle, the two others, AEC, EAC together make one right angle d; and they are equal to one another; d 32. 1, each of them therefore is half

E of a right angle.

For the same reason each of the angles CEB, EBC is half a right angle; and

F therefore the whole AEB is a right angle: and because the angle GEF is half a right angle, and EGF a right angle, for it is А

C D B equal e to the interior and oppo

e 29. 1. site angle ECB, the remaining angle EFG is half a right angle; therefore the angle GEF is equal to the angle EFG, and the side EG equal f to the side GF: again, because the angle at Bf 6. 1. is half a right angle, and FDB a right angle, for it is equal e 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 to f 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 equal & to the g 47. 1. squares of AC, 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 FG, GF are double of

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

w of EG, GF; therefore the square of EF is double of the square h 34. 1. GF; and GF is equal h 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 of AC, CD: and the square of AF is i 47.1. equal i 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: bul 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 CB, and join AE, EB ; through E draw b 31. 1. • EF parallel to AB, and through D draw DF parallel to CE:

and because the straight line EF meets the parallels EC, FD, the c 29. 1. angles CEF, EFD are 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 d 12. Ax. upon the same side less than two right angles, do meet d if pro

duced far enough: therefore EB, FD shall meet, if produced,

towards B, D: let them meet in G, and join AG: then, because e 5.1. AC is equal to CE, the angle CEA is equal e to the angle

EAC; and the angle ACE is a right angle; therefore each of the f32. 1. angles CEA, EAC is half a right angle f: for the same reason,

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