PROPOSITION V. THEOREM.

If four magnitudes are in proportion, they will be in proportion when taken inversely.

If M : N :: P: Q, then will N : M : : Q : P. For, from the given proportion, we have

MxQ=NxP, or, NxP=MxQ

Now, N and P may be made the extremes, and M and Q the means of a proportion (P. 2): hence

If four magnitudes are in proportion, they will be in proportion by composition or division.

we shall also have M±N : M :: P±Q : P. For, from the first proportion, we have

MXQ=NXP, or NXP-M× Q.

Add each of the members of the last equation to, and subtract it from MXP, and we shall have,

MXP±NX P=MXP±MXQ; or
(M±N)×P=(P±Q)×M.

But M±N and P, may be considered the two extremes, and PQ and M, the two means of a proportion (P. 2): hence,

(M±N): M :: (P±Q) : P.

PROPOSITION VII. THEOREM.

Equimultiples of any two magnitudes, have the same ratio as the magnitudes themselves.

Let M and N be any two magnitudes, and m any number whatever; then will m× M, and m×N, be equal mul

tiples of M and N: and m× M will be to m×N, in the

multiplying each member by m, and we have

mxMxN=mxNX M: then (P. 2),

mxM: mxN:: M: N.

PROPOSITION VIII. THEOREM.

Of four proportional magnitudes, if there be taken any equimultiples of the two antecedents, and any equimultiples of the two consequents, such equimultiples will be proportional.

Let M, N, P, Q, be four magnitudes in proportion ; and let m and n be any numbers whatever, then will

mxM : nx N m XP: nxQ.

For, since M :

we have

hence,

::

NP: Q2

MxQ=NxP;

m x Mx n x Q=n× N×m×P,

by multiplying both members of the equation by mXn. But m XM and n XQ, may be regarded as the two extremes, and n×N and m XP, as the means of a proportion; hence, mx M : nxN :: mxP : nxQ.

PROPOSITION IX. THEOREM.

Of four proportional magnitudes, if the two consequents be either augmented or diminished by magnitudes which have the same ratio as the antecedents, the resulting magnitudes and the antecedents will be proportional.

Let

and let

then will

M: N :: P: Q

M :

P: :: M : n:

M: Р :: N±m : Q±n.

N :: P: Q, M×Q=N×P.

For, since M: N

and since

therefore,

or

M :

Р

::

m : n, Mxn=Pxm,

MxQ±Mxn=NxP±P×m,
Mx(Q±n)=Px(N±m):

hence (P. 2), M: P:: N±m : Q±n.

PROPOSITION X. THEOREM,

If any number of magnitudes are proportionals, any one antecedent will be to its consequent, as the sum of all the antecedents to the sum of the consequents.

then, MXN+M×Q+M×S=M×N+N×P+N×R,

or,

M×(N+Q+S)=N×(M+P+R);

therefore (P. 2), M: N :: M+P+R : N+Q+S.

PROPOSITION XI. THEOREM.

If two magnitudes be each increased or diminished by like parts of each, the resulting magnitudes will have the same ratio as the magnitudes themselves.

If four magnitudes are proportional, their squares or cubes will

By squaring both members, MxQ=N°×P2,

3

3

and by cubing both members, M3× Q3=N3×P3;

Cor. In a similar way it may be shown that like powers or roots of proportional magnitudes are proportionals.

PROPOSITION XIII. THEOREM.

If there be two sets of proportional magnitudes, the products of the corresponding terms will be proportionals.

Let

and

then will

For, since

and

M : N :: P: Q,

R : S S :: T: V,

MXR NXS :: PXT : QXV.

:

MxQ=NxP,

RXV=SXT,

we shall have M×Q×R×V=N×P×S×T,

or,

therefore,

M×R× Q×V=N×S×P×T;

MXR : NXS
NXS: PXT: QXV.

PROPOSITION XIV. THEOREM.

If any number of magnitudes are continued proportionals; then, the ratio of the first to the third will be duplicate of the common ratio; and the ratio of the first to the fourth will be triplicate of the common ratio; and so on.

For, let A be the first term, and m the common ratio: the proportional magnitudes will then be represented by

3

4

A, m1×A, m3×A, m3×A, m1×A, &c.:

Now, the ratio of the first to any one of the following terms exactly corresponds with the enunciation.

BOOK III.

THE CIRCLE, AND THE MEASUREMENT OF ANGLES.

DEFINITIONS.

1. The CIRCUMFERENCE OF A CIRCLE is a curve line, all the points of which are equally distant from a point within, called the centre.

The circle is the portion of the plane terminated by the circumference.

2. Every straight line, drawn from the centre to the circumference, is called a radius, or, semidiameter. Every line which passes through the centre, and is terminated, on both sides, by the circumference, is called a diameter.

From the definition of a circle, it follows, that all the radii are equal; that all the diameters are also equal, and each double the radius.

3. Any part of the circumference is called an arc. A straight line joining the extremities of an arc, is called a chord, or subtense of the arc.*

4. A SEGMENT is the part of a circle included between an arc and its chord.

5. A SECTOR is the part of the circle included between an arc, and the two radii drawn to the extremities of the

arc.

* In all cases, the same chord belongs to two arcs, and consequently, also to two segments: but the smaller one is always meant, unless the contrary is expressed.