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XV. A circle is a plane figure contained by one line, which Book I.
is called the circumference, and is such that all straight lines drawn from a certain point within the figure to the circumference are equal to one another.
XVI. And this point is called the centre of the circle.
the centre, and terminated both ways by the circumference. XVIII. A semicircle is the figure contained by a diameter and
the part of the circumference cut off by the diameter. XIX. “ A segment of a circle is the figure contained by a
straight line, and the circumference it cuts off.”
which has three equal sides. XXV. An isosceles triangle is that which has (only) two sides
Book I. XXVI. A scalene triangle is that which has three unequal sides.
XXVII. A right angled triangle is that which has a right angle.
XXIX. An acute angled triangle is that which has three acute
angles. XXX. Of four sided figures, a square is that which has all its
sides equal, and all its angles right angles.
XXXI. An oblong is that which has all its angles right angles,
but has not all its sides equal. XXXII. A rhombus is that which has all its sides equal,
but its angles are not right angles.
XXXIII. A rhomboid is that which has its opposite sides
equal to one another, but all its sides are not equal, nor its
angles right angles. XXXIV. All other four sided figures besides these are call
ed trapeziums. XXXV. Parallel straight lines are such as are in the same
plane, and which being produced ever so far both ways, do not meet.
I. Let it be granted that a straight line may be drawn from
any one point to any other point. II. That a terminated straight line may be produced to any
length in a straight line. III. And that a circle may be described from any centre, at
any distance from that centre.
1. Things which are equal to the same are equal to one
another. VII. Things which are halves of the same, are equal to one
which exactly fill the same space, are equal to one another.
“ make the two interior angles on the same side of it taken
To describe an equilateral triangle upon a given finite straight line.
Let AB be the given straight line; it is required to describe an equilateral triangle upon it. From the centre A, at the dis
BCD, and from the centre B, at
C, in which the circles cut one b l. Post. another, draw the straight lines
CA, CB to the points A, B,
Because the point A is the centre of the circle BCD, AC is c 15. Defi- equal o to AB; and because the point B is the centre of the nition. circle ACE, BC is equal to BA: But it has been proved that
CA is equal to AB; therefore CA, CB, are each of them equal
to AB; but things which are equal to the same are equal to one d Ist Axi. another d; therefore CA is equal to CB; wherefore CA, AB,
BC are equal to one another; and the triangle ABC is therefore equilateral, and it is described upon the given straight line AB. Which was required to be done.
PROP. II. PROB.
From a given point to draw a straight line equal to a given straight line.
Let A be the given point, and BC the given straight line; it is required to draw from the point A a straight line equal to BC.
K a 1. Post. From the point A to B draw a
the straight line AB; and upon it
Н c 2. Post. DAB, and produce the straight D.
lines DA, DB; to E and F; from
the centre B, at the distance BC, d 3. Post. described the circle CGH, and
B В from the centre D, at the distance
Because the point B is the centre of the circle CGH, BC Book I. is equal to BG; and because D is the centre of the circle GKL, DL is equal to DG, and DA, DB, parts of them, are e 15. def.
. equal; therefore the remainder AL is equal to the remainder f3. Ax. BG: But it has been shown, that BC is equal to BG; wherefore AL and BC are each of them equal to BG; and things that are equal to the same are equal to one another; therefore the straight line AL is equal to BC. Wherefore, from the given point A a straight line AL has been drawn equal to the given straight line Bč. Which was to be done.
PROP. III. PROB.
From the greater of two given straight lines to cut off a part equal to the less.
Let AB and C be the two given straight lines, whereof AB is the greater. It is required to cut off from AB, the greater, a part equal to C, the less. From the point A draw a the
E B a 2. 1. straight line AD equal to C, and from the centre A, and at the distance AD, describe b the cir
F cle DEF; and because A is the centre of the circle DEF, AE shall be equal to AD; but the straight line C is likewise equal to AD; whence AE and C are each of them equal to AD; wherefore the straight line AE is equal to · C, and from AB the greater of two straight c l. Ax. lines, a part AE has been cut off equal to C the less. Which was to be done.
b 3. Post.
PROP. IV. THEOREM.
If two triangles have two sides of the one equal to two sides of the other, each to each ; and have likewise the angles contained by those sides equal to one another ; they shall likewise have their bases, or third sides, equal ; and the two triangles shall be equal ; and their other angles shall be equal, each to each, viz. those to which the equal sides are opposite.
Let ABC, DEF be two triangles, which have the two sides AB, AC equal to the two sides DE, DF, each to each, viz.