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therefore the area of a trapezoid is equal to the product of its altitude and the line joining the middle points of the sides which are not parallel.

THEOREM VI.

16. A line drawn parallel to one side of a triangle divides the

other sides proportionally.

In the triangle ABC let DE be drawn

parallel to BC; then

AE:EC= A D :D B

Draw DC and BE; the triangles A DE

and ED C, having the same vertex D, have

the same altitude; therefore (13)

B

D

A

E

C

ADE:EDC = AE :EC

And the triangles ADE and DEB, having the same vertex E, have the same altitude; therefore (13)

ADE:DEB=AD:DB

But the triangles EDC and DEB are equivalent (11), since they have the same base DE and the same altitude, viz., the perpendicular distance between the two parallels DE and BC.

Therefore (Pn. 11)

AE:EC=AD :D B

17. Corollary. As

AE:EC=AD :DB

by (Pn. 17) AE:AE+ EC = AD : AD+DB

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18. A line dividing two sides of a triangle proportionally is parallel to the third side of the triangle.

In the triangle ABC if DE divides AB and AC so that AE:EC = AD: DB, then DE is parallel to B C.

For if DE is not parallel to BC, through

D draw DF parallel to BC; then (16)

AD:DB=AF:FC

B

D

4

FE

C

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But this proportion is absurd; for AF is less than AE, while FC is greater than EC; therefore DE is parallel to BC.

19. Definition. Similar Polygons are those which are mutually equiangular, and have their homologous sides, that is, the sides including the corresponding angles, proportional.

THEOREM VIII.

20. Two triangles mutually equiangular are similar.

In the two triangles A B C, DEF, let the angle A = D, BE, and C=F; then the triangles are similar.

As the triangles are equian

gular, we have only to prove

A

B

G

H

C

E

D

F

the homologous sides proportional. Cut off AG and A H equal respectively to DE and DF, and join GIH; the triangle AGH is equal to DEF (I. 40), and the angle AGH=E; but E= B; therefore AGH = B, and GH is parallel to BC (I. 18); and (17)

or

AB:AG=AC:
AB:DE= A C :D F.

In like manner it may be proved that

AB:DE = BC : EF=AC:DF

21. Corollary. Two triangles whose sides are equally inclined to each other are similar. For if one of the triangles is

turned through an angle equal to the angle of inclination of the sides, the sides of the triangles become respectively parallel; they are therefore equiangular (I. 12) and similar (20).

THEOREM IX.

22. The altitudes of two similar triangles are proportional to the homologous sides.

Let BG and EH be the alti

tudes of the similar triangles
ABC and DEF'; then

BG:EH= A B : D E =
AC:DF=BC:EF

A

B

E

CD

F

G

H

For the two right triangles ABG, DEH are equiangular (I. 35), and similar (20); therefore BG:EH=AB : DE=AC:DF=BC:EF

THEOREM X.

23. Two triangles having an angle of the one equal to an angle of the other, and the sides including these angles proportional, are similar.

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spectively equal to DE and DF, and join GH; the triangle

AGH=DEF, and the angle AGHE (I. 40).

or

By hypothesis

AB:DE=AC:DF

AB:AG AC:AH

that is, the sides AB, AC are divided proportionally by the line GH; therefore GH is parallel to B C (18), and the angle AGH = B (I. 18); but the angle AGH = E; therefore BE, and the two triangles are mutually equiangular and therefore similar (20).

THEOREM XI.

24. In a right triangle the perpendicular drawn from the vertex of the right angle to the hypothenuse divides the triangle into two triangles similar to the whole triangle and to each other.

In the right triangle ABC if BD is drawn from the vertex B of the right angle to the hypothenuse AC, the two triangles ABD, BCD are similar to A ABC and to each other.

B

D

C

The two right triangles A B D and ABC have the acute angle A common; they are therefore equiangular (I. 35), and similar (20). The two right triangles ABC and BCD have the acute angle C common; therefore they are equiangular and similar. The two triangles A BD and BCD, being each similar to ABC, are similar to each other.

25. Cor. 1. Since ABC and ABD are similar triangles AC:AB= A B : A D

And since ABC and BCD are similar

AC:CB=CB:CD

that is, in a right triangle either side about the right angle is a mean proportional between the whole hypothenuse and the segment adjacent to that side cut off by the perpendicular drawn from the vertex of the right angle to the hypothenuse.

26. Cor. 2. As ABD and BCD are similar triangles AD:DB=DB:

that is, in a right triangle the perpendicular from the vertex of the right angle is a mean proportional between the segments of the base.

THEOREM XII.

27. The square described on the hypothenuse of a right angle is equivalent to the sum of the squares described upon the other two

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that is, the square BIKC is equivalent to the rectangle DCEL. In the same way the square AGHB can be proved equivalent to the rectangle ADLF: therefore the sum of the two rectangles, that is, the square ACEF is equivalent to the sum of the squares BIKC and AGHB; or

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