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Fifthly. The preceding equation between h, the angle A, and the opposite side a, leads to the following corresponding one between h, the angle B, and the opposite side b; sin. b = sin. h sin. B.
(292) Sixthly. From triangles COA', B'AC, and B'OC, by (4),
AC sin, COA = sin. b =
Seventhly. The preceding equation between the angle A, the opposite side a, and the adjacent side b, leads to the following corresponding one between the angle B, the opposite side b, and the adjacent side a; sin, a = cotan B tang. b.
which, substituted in (293) and (294), give
cotan. B sin. 6
sin. a =
cotan. A sin, a
sin. b =
Multiplying the first of these equations by cos. b, and the second by cos a, we have
sin, a cos. b
= cotan. B sin b,
sin. 6 cos, a = cotan, A sin. a. The product of these equations is
sin. a sin, b cos, a cos. b =cotân. A cotan. B sin. a sin. b; which, divided by sin. a sin. b, becomes
Ninthly. We have, by (288) and (292),
COS, a =
sin. B =
the product of which is, by (7) and (8),
sin. b cos, h cos, a sin. B=
cos. b sin. h
= tang. b cotan, h. But, by (289),
cos. A = tang. b cotan. h; hence cos. A = cos. a sin. B.
(296) Tenthly. The preceding equation between the side a, the opposite angle A, and the adjacent angle B, leads to the following similar one between the side b, the opposite angle B, and the adjacent angle A ; cos, B = cos. b sin. A.
(297) 11. Corollary. The ten equations, [288–297, have, by a most happy artifice, been reduced to two very simple theorems, called, from their celebrated inventor, Napier's Rules.
In these rules, the complements of the hypothenuse and the angles are used instead of the hypothenuse and the angles themselves, and the right angle is neglected.
Of the five parts, then, the legs, the complement of the hypothenuse, and the complements of the angles ; either part may be called the middle part. The two parts, including the middle part on each side, are called the adjacent parts; and the other two parts are called the opposite parts. The two theorems are as follows.
I. The sine of the middle part is equal to the product of the tangents of the two adjacent parts.
II. The sine of the middle part is equal to the product of the cosines of the two opposite parts. (B. p. 436.]
Proof. To demonstrate the preceding rules, it is only necessary to compare all the equations which can be deduced from them, with those previously obtained. [288-297.]
Let there be the spherical right triangle ABC (fig. 30) right-angled at C.
First. If co. h were made the middle part, then, by the above rule, co. A and co. B would be adjacent parts, and a and b opposite parts; and we should have
sin. (co. h)= tang. (co. A) tang. (co. B)
= cos, a cos, b;
cos. h = cos, a cos, b; which are the same as (295) and (258).
Secondly. If co. A were made the middle part; then co.h and b would be adjacent parts, and co. B and a opposite parts; and we should have
sin. (co. A) = tang. (co, 1) tang. b,
which are the same as (289) and (296).
In like manner, if co. B were made the middle part, we should have
Sides, when acute or obtuse.
cos. B = cotan. h tang. a,
cos. B = sin. A cos. b; which are the same as (290) and (297).
Thirdly. If a were made the middle part, then co. B and b would be the adjacent parts, and co. A and co. h the opposite parts; and we should have
sin. a = tang. (co. B) tang 6,
which are the same as (291) and (291).
In like manner, if b were made the middle part, we should have
sin. b = cotan. A tang. a,
which are the same as (293) and (292).
Having thus made each part successively the middle part, the ten equations, which we have obtained, must be all the equations included in Napier's Rules; and we perceive that they are identical with the ten equations [288-297].
12. Theorem. The three sides of a spherical right triangle are either all less than 90°; or else, one is less while the other two are greater than 90°, unless one of them is equal to 90°, as in 16.
Proof. When h is less than 90°, the first member of (288) is positive; and therefore the factors of its second member