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1st. The sine is positive in the first and second quadrants, and
negative in the third and fourth : 2d. The cosine is positive in the first and fourth quadrants,
and negative in the second and third :
In other words, 1st. The sine of an angle less than 180° is positive ; and the
sine of an angle greater than 180° and less than 360°, is
negative : 2d. The cosine of an angle less than 90° is positive; the cosine
of an angle greater than 90°, and less than 270°, is negative; and the cosine of an angle greater than 270°, and less than 360°, is positive.
62. The algebraic signs of the sine and cosine being determined, the signs of all the other trigonometrical functions may be at once established by means of the formulas of Table I.
Thus, for example,
Now, if the algebraic signs of sin a and cos a are alike, the tangent is positive ; if they are unlike, it is negative. Hence, the tangent is positive in the first and third quadrants, and negative in the second and fourth.
The same is also true of the cotangent: for,
the sign of the secant is always the same as that of the cosine And since,
1 cosec a
sin a the sign of the cosecant is always the same as that of the +
I t +
64. The versed sine is constantly positive. For, it is always found by subtracting the cosine from radius, and the remainder is a positive quantity, since the cosine can never exceed radius. When the cosine is negative, the versed' sine becomes greater than radius.
65. Let q denote a quadrant: then the following table will show the algebraic signs of the trigonometrical lines in the different quadrants.
First q. Second
9. Third q. Fourth 9. sine
+ 66. We have thus far supposed all angles to be estimated from the line CA from right to left, that is in the direction from A to B, to D, &c., and
B have also regarded such angles as posi. tive. It is sometimes convenient to give different signs to the angles them- D selves. If we suppose the angles to be esti
E mated from CA, in the direction from left to right, that is, in the direction from A to E, to D, &c., we must treat the angles themselves as negative, and affect them with the sign
For a negative angle less than 90°, the sine will be negative, and the cosine positive: for one greater than 90° and less than 180°, the sine and cosine will both be negative. The algebraic sign of the sine always changes, when we change the sign of the arc, but the sign of the cosine remains the same. Hence, calling x the arc, we have in general,
2 ) cos (-x) tan (-2) cot (– )
cot x. 67. We shall now examine the changes which take
sin , cOS X, tan ,
place in the values of the trigonometrical lines, as the angle increases from 0 to 360°, and shall begin with the sine and cosine.
When the arc is zero, the sine is 0, and the cosine equal to R= 1. At 90° the sine becomes equal to R= 1, and the cosine becomes 0. At 180°, the sine becomes 0, and the cosine equal to R 1. At 270°, the sine becomes equal to R =
- 1, and the cosine equal to 0. At 360°, the sine becomes equal to 0, and the cosine to R = 1. Hence,
68. By a careful consideration of the preceding principles and by making the proper substitutions in the formulas already deduced, we may now form the following Table:
sin din cota,
0, sin (180° + a) COS 0
1, cos (180° + a) tan 0
tan (180° + a) cot 0
cot a. sin (90° a)
= COS A,
sin (270° a) cosa, cos (90° a) = sin a,
cos (270° a) tan (90° a) = cot a, tan (270° – a) cot (90° a) = tan a. cot (270° a)
tan a. sin 90°
0, tan 90°
tan 270° cot 90°
0. sin (90° + a) cos a, sin (270° + a)
COS a, cos (90° + a)
cos (270° + a) tan (90° + a) cot a, tan (270° + a) cot (90° + a) tan a. cot (270° + a) tan a. sin (180° – a) sin a, sin (360°
sin (360° – a) cos (180° a) cos a, cos (360° a)
COS di, tan (180° a) tan a, tan (360° a) cot (180° – a)
cot (360° a)
cot a. sin 180° 0, sin 360°
= 0, cot 180°
sin d, cota,
69. The examinations thus far, have been limited to arcs which do not exceed 360°. It is easily shown, however, that the addition of 360° to any arc as x, will make no difference in its trigonometrical functions; for, such addition would terminate the arc at precisely the same point of the circumference. Hence, if C represent an entire circumference, or 360°, and n any. whole number, we shall have,
sin (C + x) = sin x; or, sin (n X C + x) The same is also true of the other functions.
- sip x.
70. It will further appear, that whatever be the value of an arc denoted by x, the sine may be expressed by that of an arc less than 180°. For, in the first place, we may subtract 360° from the arc 2, as often as 360° is contained in it: then denoting the remainder by y, we bave,
sin x = sin y. Then, if y is greater than 180°, make
Thus, all the cases are reduced to that in which the arc whose functions we take, is less than 180°; and since we also know that,
sin (90 + x) = sin (90 – ), they are ultimately reducible to the case of ares between 0 and 90°.
71. To find the formula for the sine of the difference of two
angles or arcs.
Let ACB be a triangle. From the vertex C let fall the perpendicular CD, on the base AB, produced.
Denote the exterior angle CBD by А B D a, and the angle CAB by b. Then, AB= AD DB. But (Art. 25), AD = AC cos b, and BD = BO cos CBD. Hence, AB AC cos b - BC cos a. Dividing both members by AB, we have
BC sin b
and AB sin ci