« ΠροηγούμενηΣυνέχεια »
SUMMARY OF THE RESULTS ARRIVED AT IN
EUCLID I. 27–48.
SECTION II. — Parallel Straight Lines and Parallelograms.
(1) The alternate angles are equal (Prop. 27).
angles (Prop. 28).
lines towards the same parts (Prop. 33). 2. Conversely, if two straight lines are parallel —
(1) The alternate angles are equal (Prop. 29).
angles (Prop. 29). 3. In any triangle(1) The exterior angle is equal to the sum of the two interior opposite
angles (Prop. 32). (2) The three interior angles are together equal to two right angles
(Prop. 32). 4. In any rectilineal figure(1) All the interior angles together with four right angles are equal to
twice as many right angles as the figure has sides (Prop. 32,
Cor. 1). (2) All the exterior angles are together equal to four right angles (Prop. 32, Cor. 2).
5. In any parallelogram the opposite sides and angles are equal (Prop. 34). B. Problem. To draw a straight line through a given point parallel to a given straight
line (Prop. 31).
SECTION III. – Areas of Parallelograms and Triangles. A. Theorems. 1. Parallelograms or Triangles on the same base or on equal bases and
between the same parallels are equal (Props. 35, 36, 37, 38). 2. Conversely, equal triangles on the same base or on equal bases and
on the saine side are between the same parallels (Props. 39, 40).
3. A parallelogram is double a triangle on the same base and between the
same parallels (Prop. 41). 4. The complements of parallelograms are equal (Prop. 43). 5. In a right-angled triangle the square on the hypotenuse is equal to the
sum of the squares on the two sides (Prop. 47). 6. Conversely, if the square on one side of a triangle is equal to the sum
of the squares on the other two sides, the triangle is right-angled
(Prop. 48). B. Problems. 1. To describe a Parallelogram equal to a given triangle or rectilineal figure,
having a given angle, and (if required) on a given straight line (Props.
42, 44, 45). 2. To describe a square on a given finite straight line (Prop. 46).
ADDITIONAL PROPOSITIONS. The following Propositions are all important, although they are not included in Euclid's Text. Many of them have been already given as Exercises or Riders ; but they are repeated here in order to bring together the results. As they are important, hints are given in all cases for their solution ; the student will, however, do well to attempt at first unaided the solution of each of these additional propositions, and if he fails he can then make use of the hints supplied.
I. — Triangles. 1. If two right-angled triangles have their hypotenuses equal, and one side
equal to one side, the triangles shall be equal in all respects.
right angles are adjacent; prove that two other sides are in one
the two triangles are equal by Prop. 26 ; or,
(2) Prove the third sides equal by Prop. 47.] 2. If two triangles have two sides of the one equal to two sides of the other,
each to each, and have also the angles opposite to one pair of equal sides
DE and BC falls along EF; if ( coincides with F the triangles are
mentary.] 3. The straight line which joins the middle points of two sides of a triangle
is parallel to the third side, and equal to half of it (see Prop. 39,
Ex. 1). 4. The straight line DE drawn through the middle point D of the side AB of
the triangle ABC, parallel to the side BC, bisects the third side AC.
ADE, CFE equal by Prop. 26.] 5. The medians of a triangle are concurrent, that is, meet at a point, and
divide each other in the ratio 2:1.
OBD, OCD equal; therefore A0, OD, and AD both bisect ABC;
therefore they are identical ; or, (2) Join AO and produce it to meet BC in X and the line BG, which
is parallel to OC, in G; CO bisects AG, therefore BOE is parallel to GC and BOCG is a parallelogram, and its diagonals bisect each other.)
6. The straight lines drawn at right angles to the sides of a triangle, from
their middle points, are concurrent.
OX; prove that OB=0A = 0C by Prop. 4, and then by Prop. 8 that
OX is at right angles to BC.] 7. The straight lines which bisect the angles of a triangle are concurrent.
[Let two bisectors meet in 0 and join OA; draw OL, OM, ON per
pendicular to the sides, and prove them equal by Prop. 26 ; prove AN e quals AM by Prop. 47, and that the angle NAO is equal to the
angle MAO by Prop. 8.] 8. The perpendiculars from the angles of a triangle on the opposite sides are
by No. 6 the perpendiculars at A, B, C are concurrent.] 9. To bisect a triangle by a straight line drawn through a given point in one
of its sides. (See Note on Additional Proposition, p. 97.) 10. To construct a right-angled triangle, having given the hypotenuse and
the sum of the remaining sides.
equal to half a right angle, and with centre B and radius equal to
required.] 11. To construct a triangle, having given the perimeter and each of the angles
at the base.
equal to half the given angles respectively ; again make the angles
in AB; PQR is the triangle required. Prove by Prop. 32.] 12. To construct a triangle, having given the base, the difference of the
angles at the base, and the difference of the remaining sides.
of base angles ; with centre A and radius equal to given difference
PAB is the triangle required.] 13. To construct a triangle, having its three medians equal to three given
given medians; finish the construction from proof (2) in No. 5.]
14. AB is a given straight line bisected at C; show that the projections of
AC, CB on any other straight line PQ are equal.
in D and E; AD and DE are equal by No. 4 on p. 101 ; and the pro
jections are equal by Prop. 34.] 15. To divide a given straight line AB into any number of equal parts.
[Draw a line AC making any angle with AB; from AC cut off any
part AP, and then parts PQ, QR, RS, ST all equal to AP; join BT, and draw lines through P, Q, R, S parallel to BT; in this case AB will be divided into five equal parts. Prove, by drawing through P, Q, R, S, lines parallel to AB, that the triangles are equal by
Prop. 26.] 16. To trisect a given finite straight line.
[As in No. 15; or describe an equilateral triangle ABC on AB;
bisect the angles CAB, CBA by the lines AO, BO, and draw OD, OE parallel to AC and BC respectively, to meet AB in D, E; prove ODE an equilateral triangle by Props. 29 and 32; and then prove
DA DO = DE by Prop. 29.] 17. To find a point in a given straight line such that the sum of its distances
from two given fixed points, on the same side of the line, may be the
perpendicular to an equal length on the other side of the line ;
its distances greater by Props. 4 and 20.] 18. A and B are two fixed points and PQ a given straight line ; find in point X
in PQ such that the angle AXP is equal to the angle BXQ. Does it make any difference whether the points A and B are on the same or opposite sides of PQ?
[Same as No. 17.) 19. Through a given point P to draw a straight line, such that the part of
it intercepted between two given lines AB, AC may be bisected at P.
another; EP produced is the line required. Prove by Prop. 26 as
in No. 4.] 20. To draw a straight line at night angles to a given straight line from the
extremity of it and without producing the given line,
[See Ex. 4, p. 69.]