Therefore the ▲ ABE ▲ FGK: AB2: FG2, and ABCD : ▲ GHI :: BC2 GH2;

and ABDE: A GIK :: DE2: IK2.

But as the two polygons are similar, their like sides are proportional, and consequently their squares also proportional; so that all the ratios, AB to FG2, and BC2 to GH2 and DE2 to IK2, are equal among themselves, and consequently the corresponding triangles also, ABE to FGK, and BCD to GHI, and BDE to GIK, have all the same ratio, viz. that of AB2 to FG2: and hence all the antecedents, or the figure ABCDE, have to all the consequents, or the figure FGHIK, still the same ratio, viz. that of AB to FG2 (th. 72).

Q. E. D.

THEOREM XC.

Similar Figures Inscribed in Circles have their Like Sides, and also their Whole Perimeters, in the Same Ratio as the Diameters of the Circles in which they are Inscribed.

other figure, or the whole perimeter AB + BC + &c. of the one figure, to the whole perimeter FG+ GH+ &c. of the other figure, as the diameter AL to the diameter FM.

For, draw the two corresponding diagonals, AC, FH, as also the lines BL, GM. Then, since the polygons are similar, they are equiangular, and their like sides have the same ratio (def. 70); therefore the two triangles ABC, FGH, have the angle B the angle G, and the sides AB, BC, proportional to the two sides FG, GH, consequently these two triangles are equiangular (th. 86), and have the angle ACB = FHG. But the angle ACB ALB, standing on the same arc AB; and the angle FнG FMG, standing on the same arc FG; therefore the angle ALB = FMG (th. 1. And since the angle ABL = FGM, being both right angles, because in a semicircle; therefore the two triangles, ABL, FGM, having two angles equal, are equiangular; and conséquently their

like sides are proportional (th. 84); hence AB FG :: the diameter AL: the diameter Fм.

In like manner, each side BC, CD, &c. has to each side GH, HI, &c. the same ratio of AL to FM and consequently the sums of them are still in the same ratio; viz. AB+BC + CD, &c. FG+CH + HI, &c. :: the diam. AL: the diam. FM (th. 72).

Q. E. D.

THEOREM XCI.

Similar Figures Inscribed in Circles, are to each other as the Squares of the Diameters of those Circles.

the polygon FGHIK, as AL2 to FM2.

M

H

But, by

For, the figures being similar, are to each other as the squares of their like sides, AB2 to FG2 (th. 88). the last theorem, the sides AB, FG, are as the diameters AL, FM; and therefore the squares of the sides AB2 to FG2, as the squares of the diameters AL2 to FM2 (th. 74). Consequently the polygons ABCDE, FGHIK, are also to each other as the squares of the diameters AL2 to Fм2 (ax. 1).

Q. E. D.

THEOREM XCII.

The Circumferences of all Circles are to each other as their Diameters.

Let D, d, denote the diameters of two circles, and c, c, their circumferences;

then will Dd: cc, or D c d : c.

For, (by theor. 90,) similar polygons inscribed in circles have their perimeters, in the same ratio as the diameters of those circles.

Now, as this property belongs to all polygons, whatever the number of the sides may be; conceive the number of the sides to be indefinitely great, and the length of each indefinitely small, till they coincide with the circumference of

the

the circle, and be equal to it, indefinitely near. Then the perimeter of the polygon of an infinite number of sides, is the same thing as the circumference of the circle. Hence it appears that the circumferences of the circles, being the same as the perimeters of such polygens, are to each other in the same ratio as the diameters of the circles.

THEOREM XCIII.

Q. E. D.

The Areas or Spaces of Circles, are to cach other as the Squares of their Diameters, or of their Radii.

LET A, a, denote the areas or spaces of two circles, and D, d their diameters; then A: a :: D2 : d2.

For (by theorem 91 similar polygons inscribed in circles are to each other as the squares of the diameters of the circles.

Hence, conceiving the number of the sides of the polygons to be increased more and more, or the length of the sides to become less and less, the polygon approaches nearer and nearer to the circle, till at length, by an infinite approach, coincide, and become in effect equal; and then it follows that the spaces of the circles, which are the same as of the polygons, will be to each other as the squares of the diameters of the circles.

Q. E. D.

Corol. The spaces of circles are also to each other as the squares of the circumferences; since the circumferences are in the same ratio as the diameters (by theorem 92).

THEOREM XCIV.

The Area of any Circle, is Equal to the Rectangle of Half its Circumference and half its Diameter.

CONCEIVE a regular polygon to be inscribed in the circle and radii drawn to all the angular points, dividing it into as many equal triangles as the polygon has sides, one of which ABC, of which the altitude is the perpendicular CD from the centre to the base AB.

Then the triangle ABC, being equal to

AD

a rectangle of half the base and equal altitude (th. 26, cor. 2). is equal to the rectangle of the half base AD and the altitude CD ;

conse

consequently the whole polygon, or all the triangles added together which compose it, is equal to the rectangle of the common altitude CD, and the halves of all the sides, or the half perimeter of the polygon.

A

Now, conceive the number of sides of the polygon to be indefinitely increased; then will its perimeter coincide with the circumference of the circle, and consequently the altitude CD will become equal to the radius, and the whole polygon equal to the circle. Consequently the space of the circle, or of the polygon in that state, is equal to the rectangle of the radius and half the circumference.

Q. E. D.

OF PLANES AND SOLIDS.

DEFINITIONS.

DEF. 88. The common Section of two Planes, is the line in which they meet, to cut each other.

89. A Line is Perpendicular to a Plane, when it is perpendicular to every line in that plane which meets it.

90. One Plane is Perpendicular to Another, when every line of the one, which is perpendicular to the line of their common section, is perpendicular to the other.

91. The inclination of one Plane to another, or the angle they form between them, is the angle contained by two lines drawn from any point in the common section, and at right angles to the same, one of these lines in each plane.

92. Parallel Planes, are such as being produced ever so far both ways, will never meet, or which are every where at an equal perpendicular distance.

93. A Solid Angle, is that which is made by three or more plane angles, meeting each other in the same point.

94. Similar

94. Similar Solids, contained by plane figures, are such as have all their solid angles equal, each to each, and are bounded by the same number of similar planes, alike placed.

95. A Prism, is a solid whose ends are parallel, equal, and like plane figures, and its sides, connecting those ends, are parallelograms.

96. A Prism takes particular names according to the figure of its base or ends, whether triangular, square, rectangular, pentagonal, hexagonal, &c.

97. A Right or Upright Prism, is that which has the planes of the sides perpendicular to the planes of the ends or base.

98. A Parallelopiped, or Parallelopipedon, is

a prism bounded by six parallelograms, every opposite two of which are equal, alike, and parallel.

99. A Rectangular Parallelopipedon, is that whose bounding planes are all rectangles, which are perpendicular to each other.

100. A Cube, is a square prism, being bounded by six equal square sides or faces, and are perpendicular to each other.

101. A Cylinder, is a round prism, having circles for its ends; and is conceived to be formed by the rotation of a right line about the circumferences of two equal and parallel circles, always parallel to the axis.

102. The Axis of a Cylinder, is the right line joining the centres of the two parallel circles, about which the figure is described.

103. A Pyramid, is a solid, whose base is any right-lined plane figure, and its sides triangles, having all their vertices meeting together in a point above the base, called the Vertex of the pyramid.

104. A pyramid, like the prism, takes particular names from the figure of the base.

105. A Cone, is a round pyramid, having a circular base, and is conceived to be generated by the rotation of a right line about the circumference of a circle, one end of which is fixed at a point above the plane of that circle.

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44

106. The