Proposition 21. Theorem.*

551. Between two straight lines not in the same plane one, and only one, common perpendicular can be drawn. Hyp. Let AB and CD be the

AB' to MN, and let their intersection A'B' meet CD at A'.

Then A'B' is || to AB.

At A' erect A'A in the plane AB' 1 to A'B'.

(517)

(537)

(487)

(71)

If there is any other common to AB and CD, let it be HG.

Because HG is to AB,

... it is to a line HL drawn || to AB in the plane

MN,

and ... HG is to the plane MN.

(71)

(500)

But GK in the plane AB', I to A'B', is to MN. (537) .. from the pt. G there are two Ls GK, GH to MN.

But this is impossible.

(502)

... HG is not a common to the lines AB, CD.

Q.E.D.

... A'A is the only common to AB and CD. 552. COR. The perpendicular A'A is the least distance between AB and CD; for any other line GH > GK (497), or its equal A'A.

EXERCISES.

1. Parallel lines intersecting the same plane make equal angles with it. (492)

2. If a plane bisects a line perpendicularly, every point of the plane is equally distant from the extremities of the line.

3. If three lines in space are parallel, in how many planes may they lie when taken two at a time?

4. If four lines in space are parallel, in how many planes may they lie when taken two at a time?

5. How many different planes may the sides of a quadrilateral in space contain when taken two and two?

6. If two lines not in the same plane are intersected by the same line, how many planes may be determined by the three lines taken two and two?

7. A straight line makes equal angles with parallel planes.

8. The sum of two adjacent diedral angles, formed by one plane meeting another, is equal to two right diedral angles.

9. If two planes intersect each other, the opposite or vertical diedral angles are equal.

10. When a plane intersects two parallel planes, the alternate-interior diedral angles are equal, and the exteriorinterior diedral angles are equal.

11. Show that two observations with a spirit-level are sufficient to determine if a plane is horizontal: and prove that for this purpose the two positions of the level must not be parallel.

12. To draw a straight line perpendicular to a given plane from a given point outside of it.

13. To draw a straight line perpendicular to a given plane from a given point in the plane,

POLYEDRAL ANGLES.

DEFINITIONS.

553. When three or more planes meet in a common point, they are said to form a polyedral angle at that point.

The common point in which the planes meet is the vertex of the angle, the intersections of the planes are the edges, the portions of the planes between the edges

are the faces, and the plane angles formed by the edges are the face-angles:

Thus, the point S is the vertex, the straight lines SA, SB, etc., are the edges, the planes A SAB, SBC, etc., are the faces, and the angles ASB, BSC, etc., are the face-angles of the polyedral angle S- ABCD.

S

554. The edges of a polyedral angle may be produced indefinitely; but to represent the angle clearly, the edges and faces are supposed to be cut off by a plane, as in the figure above. The intersection of the faces with this plane forms a polygon, as ABCD, which is called the base of the polyedral angle.

555. In a polyedral angle, each pair of adjacent faces forms a diedral angle, and each pair of adjacent edges forms a face-angle. There are as many edges as faces, and therefore as many diedral angles as faces.

556. The magnitude of a polyedral angle depends only upon the relative position of its faces, and is independent of their extent. Thus, by the face SAB is not meant the triangle SAB, but the indefinite plane between the edges SA, SB produced indefinitely.

557. Two polyedral angles are equal, when the face and

diedral angles of one are respectively equal to the face and diedral angles of the other, taken in the same order.

558. A polyedral angle of three faces is called a triedral angle; one of four faces is called a tetraedral angle; etc. 559. A polyedral angle is convex when its base is a convex polygon. (141)

560. A triedral angle is called isosceles when it has two of its face-angles equal; when it has all three of its faceangles equal it is called equilateral.

561. A triedral angle is called rectangular, bi-rectangular, or tri-rectangular, according as it has one, two, or three, right diedral angles.

The corner of a cube is a tri-rectangular triedral angle.

562. Two polyedral angles are symmetrical, when the face and diedral angles of one are equal to the face and diedral angles of the other, each to each, but arranged in reverse order.

S'

Thus, the triedral angles S- ABC, S'- A'B'C' are symmetrical when the face-angles ASB, BSC, CSA are equal respectively to the face-angles A'S'B', B'S'C', C'S'A', and the diedral angles SA, SB, SC to the diedral angles S'A', S'B', S'C'.

When two polyedral angles are

A

Α'

symmetrical, it is, in general, impossible to bring them into coincidence.

The two hands are an illustration. The right hand is symmetrical to the left hand, but cannot be made to coincide with it. The right glove will not fit the left hand, but is symmetrical to it.

563. Opposite or vertical polyedral angles are those in which the edges of one are the prolongations of the edges. of the other.

Hyp. Let S - ABC, S- A'B'C' be two opp. triedrals.

To prove they are symmetrical.

Proof. Because the faces ASB and A'SB' are vertical

... the diedral s whose edges are SA, SB, etc., = respectively the diedrals whose edges are SA', SB', etc. But the edges of SA'B'C' are arranged in the reverse order from the edges of S- ABC.

.. S-ABC is symmetrical to S-A'B'C'.

EXERCISE.

Q. E.D.

Pass two parallel planes, one through each of two straight lines which do not meet and are not parallel.

Let AB, CD be the lines: draw AE | to CD, CF || to AB. .. plane AEB is || to plane CFD.