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For (Art. 30.) sin a cos b=1 sin (a + b)+sin (a-b)

sin a sin b=1 cos (a - b)- {cos (a+b).

And these equations, if a=30°, and therefore, sin a=1, become cos b=sin (30° +b) + sin (30°— b)

sin b=cos (30°—b)-cos (30° +b) ::. sin (30° +b)=cos b-sin (30°b)

cos (30° + b)=cos (30° – b) - sin b.

So that if for b be substituted, succcssively, all the arches. between 0° and 30°, there will be found the sines and cosines of all arches between 30° and 60°: and amongst the sines and cosines of all the arches between oo and 45° will be found the sines and cosines of all arches whatever.

Lastly, if the tangents and co-tangents be computed, of all arches between 0° and 30°, the tangents and cotangents of all arches whatever, that are measured by an even number of seconds may be found, without any other arithmetical operation, than a subtraction, and a division by 2. For (Art. 30.) cot. 2a= ž (cot. a — tan. a). If, therefore, a=30°— b, and 2a = 60° — 2b, and cot (60° – 26) = tan (30° + 2b)=

cot (30° b) — tan (30° – b)

2

Whence, the tangents and co-tangents of all arches between 30° and 60° may be found from the tangents and co-tangents of arches that are less than 30°; and thus the tangents and co-tangents of all arches whatever

will be known, when those of all arches from 0° to 45€ have been found.

But it is manifest, that a and b in the two forms,
cot 20 = (cot a tan a)

cot (30°—b) – tan (30°– 6)
tan (30° +26) =

2

denote whole numbers of seconds; and therefore, the arch (30° + 2b) of which the tangent is thus found, must always consist of an even number of seconds.

The considerations, however, which have been suggested, in this last article, indicate a very great abridgement of labour in the actual computation of the trigonometrical functions of circular arches.

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TREATISE

ON

Spherics.

PART II.

SPHERICAL TRIGONOMETRY.

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ON THE INVESTIGATION OF SUCH GENERAL PROPOSITIONS

AS ARE APPLICABLE TO THE PURPOSES OF SPHERICAL

TRIGONOMETRY.

DEFINITION.

(227.) SPHERICAL Trigonometry is that part of Spherics, in which are investigated the relations existing between the sides, the angles, and the surface of a spherical triangle, with a view to the resolution of the following general problem : “ Of the six first quantities, namely, those made up of the three sides, and the three angles, of a proposed spherical triangle, any three being given, to determine the rest *."

* That this problem involves no impossibility, is very evident, from what has been already proved, in treating of Spherical Geometry:

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