3. Rules for the Twelve Cases of Oblique Angled Spherical Triangles.

Fig. 26, 27.

Given.

Case.

Sought.

Rule.

1.

Two sides AB, AC, and the included

angle A.

One of the Draw the perpendicular CD from C the unknown
other angles angle, not required, on AB.
B. R:cos A : : tan AC : tan AD, less or greater than 90°,
as A and AC are of the some or of different affection.

BD=AB, AD.
sin BD: sin AD: : tan A : tan B, of the same or of

different affection, as AD is less or greater than AB. The third cos AD: cos BD: : cos AC : cos BC, less or greater side BC. than 90°, as A and BD are of the same or of different

affection.

2.

5.

:

The angle B, sin BC : sin AC : : sin A : sin B. The affection of B opposite to the is doubtful; unless it can be determined by this rule, other given that according as AC+BC is greater or less than 180°,

side AC A+B is also greater or less than 180°. The included R:cos AC:: tan A : cot ACD, less or greater than 90°, angle ACB. as AC and A are of the same or of different affection.

tan BC: tan AC :: cos ACD : cos BCD greater than

90°, if one or all of the 3 terms ACD, AC, BC, are

greater than 90°; otherwise less than 90°. ACD + BCD=ACB, doubtful. If ACD+BCD ex

ceed 180°, take their difference; if BCD is greater

than ACD, take their sum, for the angle ACB. The third R: cos A : : tan AC : tan AD, less or greater than 90°, side AB. as A and AC are of the same or of different affection.

cos AC : cos BC :: cos AD: cos BD, greater than 90°,

7.

Given.

Case.

Sought.

Rule.

8.

1

The side BC sin B : sin A : : sin AC : sin BC, doubtful; unless it
opposite to the can be determined by this rule, that according as
other given A+B is greater or less than 180°, AC+BC is also
angle A.

greater or less than 180°.
The side AB R : cos A : : tan AC : tan AD, less or greater than 90°,
adjacent to the as A and AC are of the same or of different affection,
given angles tan B : tan A : : sin AD: sin BD, doubtful. AB=
A, B. AD+BD. If AD+BD be greater than 180°, take

their difference ; if B D is greater than AD, take their
sum for AB.

10.

4. Rules for the first Ten Cases of Oblique Angled Spherical Triangles,

expressed in general terins.

1. One of the Find an arc x, so that
other angles. rad xcos given angle

=tan x;
cot side opposite angle sought
and let y=difference between x and the other given side,

tan given angle X sin x.
then will

=tan angle sought.

sin y

Two sides and the included

angle.

2
.

Given. Case.

Rule.

3.

Sought.
The third

angle.

Find an arc I, so that
radXcos given side

cot r ; and
cot one of the given angles
let y=difference between x and the other given angle,

cos first angle X sin y then will

=cos angle sought. sin I

Two angles and the side between

them.

4. One of the Find an arc 3, so that
other sides. radXcos given side

=cots; and
cot angle opposite side sought
let y=difference between x and the other given angle,

cos xXtan given side then will

=lan side sought.

5. The angle
opposite to sin side op. to ang. soughtX

sin giv, ang: =sin ang. sought the other gi- sin side opposite to given angle

ven side.

given sides:

Two sides and an angle opposite to one of them.

6. The angle, Find an arc x, so that
included be-radXcos side adjacent to given angle

Ecot ;
tween
the

cot given angle
and another y, so that
tan same side Xcos x

=cos Y ;
tan other side

then will x+y=angle sought.
7. The third Find an arc a, so that
side.
rad Xcos given angle

=lan I ;
cot given side adjacent given angle

and another y, so that
cos side opposite given angle x cos x

Ecos y ;
cos other given side

then will x+y=side sought.
8. The side op-
posite to the sin ang. op. to side soughtX sin giv. side

-sin side sought.
other given sin angle opposite to given side
angle.

=lan I;

two

Two angles and a side opposite to one of them.

=sin yi

9. The side ad- Find an arc x, so that
jacent to the radXcos given angle adjacent to given side
given

cot given side
angles. and another y, so that

tan same angle X sin x

tan other angle

then will x+y=side sought.
10. ! The third Find an arc x, so that
angle. rad Xcos given side

cot I;
cot angle adjacent to it

and another y, so that
cos angle opposite to given side X sin z

=sin
cos other given angle
then will x+y=angle sought.

y ;

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Edinburgh.