THEOREM V.

In every rectilineal triangle, the sum of two sides is to their difference as the tangent of half the sum of the angles opposite those sides, to the tangent of half their difference.

For, AB: BC:: sin C: sin A (Theorem III.). Hence, AB+BC: AB-BC :: sin C+sin A: sin C-sin A. But

sinC+sin A: sin C-sin A:: tang

C+A

B

2

the property we had to demonstrate.

With the aid of these five theorems we can solve all the cases of rectilineal trigonometry.

Scholium. The required part should always be found from the given parts; so that if an error is made in any part of the work, it may not affect the correctness of that which follows.

SOLUTION OF RECTILINEAL TRIANGLES BY MEANS OF

LOGARITHMS.

It has already been remarked, that in order to abridge the calculations which are necessary to find the unknown parts of a triangle, we use the logarithms of the parts instead of the parts themselves.

Since the addition of logarithms answers to the multiplication of their corresponding numbers, and their subtraction to the division of their numbers; it follows, that the logarithm of the fourth term of a proportion will be equal to the sum of the logarithms of the second and third terms, diminished by the logarithm of the first term.

Instead, however, of subtracting the logarithm of the first term from the sum of the logarithms of the second and third terms, it is more convenient to use the arithmetical complement of the first term.

The arithmetical complement of a logarithm is the number which remains after subtracting the logarithm from 10. Thus 10-9.274687=0.725313: hence, 0.725313 is the arithmetical complement of 9.274687.

Q

It is now to be shown that, the difference between two logarithms is truly found, by adding to the first logarithm the arithmetical complement of the logarithm to be subtracted, and diminishing their sum by 10.

Let

a the first logarithm.

b= the logarithm to be subtracted.

c = 10-b-the arithmetical complement of b.

Now, the difference between the two logarithms will be expressed by a-b. But from the equation c-10-b, we have c—10——b: hence if we substitute for -b its value, we shall have

a-b=a+c-10,

which agrees with the enunciation.

When we wish the arithmetical compleinent of a logarithm, we may write it directly from the tables, by subtracting the left hand figure from 9, then proceeding to the right, subtract each figure from 9, till we reach the last significant figure, which must be taken from 10: this will be the same as taking the logarithm from 10.

Ex. From 3.274107 take 2.104729.

jecting the 10.

We therefore have, for all the proportions of trigonometry, the following

RULE.

Add together the arithmetical complement of the logarithm of the the first term, the logarithm of the second term, and the logarithm of the third term, and their sum after rejecting 10, will be the logarithm of the fourth term. And if any expression occurs in which the arithmetical complement is twice used, 20 must be rejected from the sum.

SOLUTION OF RIGHT ANGLED TRIANGLES.

a

C

C

A

Let A be the right angle of the proposed right angled triangle, B and C the other two angles; let a be the hypothenuse, b the side opposite the angle B, c the side opposite the angle C. Here we must consider that the B two angles C and B are complements of each other; and that consequently, according to the different cases, we are entitled to assume sin C=cos B, sin B=cos C, and likewise tang B= cot C, tang C=cot B. This being fixed, the unknown parts of a right angled triangle may be found by the first two theorems; or if two of the sides are given, by means of the property, that the square of the hypothenuse is equal to the sum of the squares of the other two sides.

EXAMPLES.

Ex. 1. In the right angled triangle BCA, there are given the hypothenuse a=250, and the side b=240; required the other parts.

or,

R sin B: a b (Theorem I.).
a: b

:

: R: sin B.

When logarithms are used, it is most convenient to write the proportion thus,

To sin B

73° 44′ 23′′ (after rejecting 10) 9.982271

But the angle C=90°-B=90°-73° 44′ 23′′=16° 15′ 37′′ or, C might be found by the proportion,

As hyp. a
To side b
So is R

Or the side c might be found from the equation

Ex. 2. In the right angled triangle BCA, there are given, sideb=384 yards, and the angle B=53° 8′ : required the other parts.

Note. When the logarithm whose arithmetical complement is to be used, exceeds 10, take the arithmetical complement with reference to 20 and reject 20 from the sum.

To find the hypothenuse a.

R: sin B: a b (Theorem I.). Hence,

Ex. 3. In the right angled triangle BAC, there are given,

side c=195, angle B=47° 55',

required the other parts.

Ans. Angle C-42° 05′, a=290.953, b=215.937.

SOLUTION OF RECTILINEAL TRIANGLES IN GENERAL.

Let A, B, C be the three angles of a proposed rectilineal triangle; a, b, c, the sides which are respectively opposite them; the different problems which may occur in determining three of these quantities by means of the other three, will all be reducible to the four following cases.

CASE I.

Given a side and two angles of a triangle, to find the remaining parts.

First, subtract the sum of the two angles from two right angles, the remainder will be the third angle. The remaining sides can then be found by Theorem III.

I. In the triangle ABC, there are given the angle A=58° 07′, the angle B=22° 37′, and the side c=408 yards: required the remaining angle and the two other sides.

This angle being greater than 90° its sine is found by taking that of its supplement 80° 44'.

2. In a triangle ABC, there are given the angle A=38° 25′ B=57° 42', and the side c=400: required the remaining parts.

Ans. Angle C-83° 53′, side a=249.974, side b-340.04.

CASE II.

Given two sides of a triangle, and an angle opposite one of then, to find the third side and the two remaining angles.