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makes right angles with GH, that is, with any straight line drawn through E in the plane passing through AB, CD. In like manner, it may be proved, that FE makes right angles with every straight line which meets it in that plane. But a straight line is at right angles to a plane when it makes right angles with every straight line which meets it in that plane* : therefore *3 Def. 11. EF is at right angles to the plane in which are AB, CD. Wherefore, if a straight line, &c.

Q. E. D.

PROP. V. THEOR.

If three straight lines meet all in one point, and a straight See N.

line stand at right angles to each of them in that point ; these three straight lines are in one and the same plane.

Let the straight line AB stand at right angles to each of the straight lines BC, BD, BE, in B the point where they meet: BC, BD, BE, shall be in one and the same plane.

If not, let, if it be possible, BD and BE be in one plane and BC be above it; and let a plane pass through AB, BC, the common section of which, with the plane in which BD and BE are, is a straight* line; * 3. 11. let this be BF: therefore the three straight lines, AB, BC, BF, are all in one plane, viz. that which passes through AB, BC: and because AB stands at right angles to each of the straight lines BD, BE, it is also at right angles* to the plane passing through them; * 4. 11. and therefore makes right angles* with every straight * 3 Def. 11. line in that plane which meets it: but BF, which is in that plane, meets it; therefore the angle ABF is a right angle: but the angle A ABC, by the hypothesis, is also a right angle; therefore the angle ABF is equal to the angle ABC, and they

-D are both in the same plane, which ist impossible; therefore the straight line BC is not above the plane in which are BD and BE: wherefore the three straight lines BC, BD, BE are in one and the same plane. Therefore, if three straight lines, &c. Q. E. D.

E 9 Ax.

N

PROP. VI. THEOR.

† 11. 1. + 3. 1.

D

E

If two straight lines be at right angles to the same plane,

they shall be parallel to one another.
Let the straight lines AB, CD, be at right angles to
the same plane: AB shall be parallel to CD.

Let them meet the plane in the points
B, D, and draw the straight line BD, to

C
which draw+ DE at right angles, in the
same plane; and maket DE equal to AB,
and join BE, AE, AD. Then, because

AB is perpendicular to the plane, it shall * 3 Def. 11. make right* angles with every straight

line which meets it, and is in that plane:
but BD, BE, which are in that plane, do each of them
meet AB; therefore each of the angles ABD, ABE is
a right angle: for the same reason, each of the angles
CDB, CDE is a right angle: and because AB is equal
to DE, and BD common, the two sides AB, BD are
equal to the two ED, DB, each to each; and they con-
tain right angles: therefore the base AD is equal* to
the base BE: again, because AB is equal to DE, and
BE to AD; AB, BE are equal to ED, DA each to
each; and, in the triangles, ABE, EDA, the base
AE is common: therefore the angle ABE is equal*
to the angle EDA: but ABE is a right angle; there-
fore EDX is also a right angle, and ED perpendicular
to DA: but it is also perpendicular to each of the two
BD, DC; wherefore ED is at right angles to each of
the three straight lines BD, DA, DC in the point in
which they meet; therefore* these three straight lines
are all in the same plane: but AB is in the plane in
which are BD, DA*, because any three straight lines
which meet one another are in one plane: therefore
AB, BD, DC, are in one plane: and each of the
angles ABD, BDC is a right angle; therefore AB is
parallel* to CD. Wherefore if two straight lines, &c.

* 4. 1.

* 8. 1.

* 5. 11.

* 2. 11.

28. 1.

Q. E. D.

PROP. VII. THEOR.

See N.

If two straight lines be parallel, the straight line drawn

from any point in the one to any point in the other, is in the same plane with the parallels.

Let AB, CD be parallel straight lines, and take any point E in the one, and the point F in the other: the straight line which joins E and F shall be in the same plane with the parallels. If not, let it be, if possible, above

B the plane as EGF; and in the plane ABCD in which the parallels are,

G

H draw the straight line EHF from E to F: and since EGF also is a straight line, the two straight lines EHF, EGF, include a space between them, which is * impossible. * 10 Ax. 1.

Therefore the straight line joining the points E, F is not above the plane in which the parallels AB, CD are, and is therefore in that plane. Wherefore, if two straight lines, &c.

Q. E. D.

PROP. VIII. THEOR.

If two straight lines be parallel, and one of them is at See N.

right angles to a plane; the other also shall be at right angles to the same plane.

Let AB, CD be two parallel straight lines, and let one of them. AB be at right angles to a plane: the other CD shall be at right angles to the same plane.

Let AB, CD meet the plane in the points B, D, and join BD: therefore* AB, CD, BD, are in one plane. * 7. 11. In the plane to which AB is at right angles, drawt + 11. 1. DE at right angles to BD, and maket DE equal to + 3. 1. AB, and join BE, AE, AD. And because AB is perpendicular to the plane it is perpendicular to every straight line which meets it, and is in that plane*; *3 Def.11. therefore each of the angles ABD, ABE is a right angle: and because the straight line BD meets the parallel straight lines AB, CD, the angles ABD, CDB are together equal* to two right angles: and ABD is * 29. 1. a right angle; therefore also CDB is a right angle, and CD perpendicular to AN BD: and because AB is equal to DE, and BD common, the two AB, BD are equal to the two ED, DB, each to each; B and the angle ABD is equal to the angle EDB, because each of them is a right angle; therefore the base AD is equal* to

* 4.1.

* 8.1.

+ Constr.

* 4. 11.

the base BE: again, because AB is equal to DE, and BE to AD, the two AB, BE, are equal to the two ED, DA, each to each ; and the base AE is common to the triangles ABE, EDA; wherefore the angle ABE is equal* to the angle EDA: but ABE is a right angle; and therefore EDA is a right angle, and ED perpendicular to DA: but it is also perpendicular tot BD; there

fore ED is perpendicular* to the plane which passes *3 Def. 11. through BD, DA; and therefore* makes right angles

with every straight line meeting it in that plane: but DC is in the plane passing through BD, DA, because all three are in the plane in which are the parallels, AB, CD; wherefore ED is at right angles to DC; and therefore CD is at right angles to DE: but CD is also at right angles to DB; therefore CD is at right angles to the two straight lines DE, DB in the point of their intersection D; and therefore is at right angles* to the plane passing through DE, DB, which is the same plane to which AB is at right angles. Therefore, if two straight lines, &c.

* 4. 11.

Q. E. D.

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Two straight lines which are each of them parallel to the

same straight line, and not in the same plane with it, are parallel to one another.

| 11. 1.

B В

E

-F

Let AB, CD be each of them parallel to EF, and not in the same plane with it: AB shall be parallel to CD.

In EF take any point G, from whicht
draw, in the plane passing through

AH
EF, AB, the straight line GH at right
angles to EF; and in the plane pass-
ing through EF, CD, draw GK at

CK
right angles to the same EF. And
because EF is perpendicular both to GH and GK, EF
is perpendicular* to the plane HGK passing through
them: and EF is parallel to AB; therefore AB is '
at right angles* to the plane HGK. For the same
reason CD is likewise at right angles to the plane HGK.
Therefore, AB, CD, are each of them at right angles
to the plane HGK. But if two straight lines are at
right angles to the same plane, they are parallel* to

* 4. 11.

* 8. 11.

* 6. 11.

one another: therefore AB is parallel to CD. Wherefore, two straight lines, &c. R. E. D.

PROP. X. THEOR.
If two straight lines meeting one another be parallel to

two others that meet one another, and are not in the
same plane with the first two; the first two and the
other two shall contain equal angles.

Let the two straight lines AB, BC, which meet one another, be parallel to the two straight lines DE, EF, that meet one another, and are not in the same plane with AB, BC: the angle ABC shall be equal to the angle DEF.

Take BA, BC, ED, EF, all equal to one another; and join AD, CF, BE, AC, DF: then because BA is equal and parallel to ED, therefore AD is * both equal • 53. 1. and parallel to BE. For the same reason, CF is equal and parallel to BE. Therefore AD and CF are each of them equal and parallel to BE. But straight lines that are parallel to the same straight line, and not in

• 9. 11. the same plane with it, are parallel to one another; therefore AD is parallel to B

• 1 Ax. 1. CF; and it is equal* to it; and AC, DF

A A

C join them towards the same parts; and therefore * AC is equal and parallel to DF. And because AB, BC are equal to DE, EF, each to each, and the base AC to the base DF; the angle ABC is equal* to the angle DEF. Therefore, if two straight lines, &c.

Q. E. D.

• 33. 1.

• 8. 1.

PROP. XI. PROB.
To draw a straight line perpendicular to a plane, from a

given point above it.
Let A be the given point above the plane BH; it
is required to draw from the point A a straight line
perpendicular to the plane BH.

In the plane draw any straight line BC, and from the point A draw* AD perpendicular to BC. If then * 12. 1. AD be also perpendicular to the plane BH, the thing required is already done; but if it be not, from the point D draw *, in the plane BH, the straight line * 11. 1. DE, at right angles to BC; and from the point A draw

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