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Hence the following rules :
1. To convert grades into degrees.

From the number of grades SUBTRACT y, and the remainder is the number of degrees.

2. To convert degrees into grades.

To the number of degrees ADD }, and the sum is the number of grades. Ex. 1. Convert 139 18' 75" into English measure. No. of grades =

= 13.1875 Subtract of this 1:31375 :: No of degrees 11.86875

60 521-12500

60

71.500 Ans. 11° 52' 711.5.

Ex. 2. Convert 18° 7' 30" into French measure.
No. of degrees

= 18.125
Add ; of this 2.0138
:: No. of grades 20.1388

Ans. 20° 13' 88.g.

=

3. An angle may be conceived to be generated by the revolution of a line about a fixed point. Thus—

Let OA be an initial line, and let a line, OP, starting from OA, revolve with O as centre, and take up successively the positions OP, OP, OP, OP.

Р.

BI

Now the magnitude of an angle may be measured by the amount of turning required to generate it. When, therefore, the revolving line reaches the position OB, we may conceive an angle to have been generated whose magnitude is two right angles. And, further, when the revolving line assumes the positions OP, OP, the angles AOP3, AOP, (the letters being read in the direction of revolution) are angles, whose magnitudes are each greater than two right angles. Indeed, when the revolving line again reaches the position OP, we may conceive an angle to have been generated whose magnitude is four right angles. Lastly, if the revolution of the line OP be continued, we may conceive of angles being generated to whose magnitude there is no limit.

Ex. I. 1. Express 39° 22' 30" in French measure, and 139 150 75" in English measure.

2. One of the angles at the base of an isosceles triangle is 50°. Express the vertical angle in grades.

3. Divide an angle of n degrees into two such parts that the number of degrees in one part may be twice the number of grades in the other.

4. Two angles of a triangle are respectively a', b', express the other angle in degrees and grades.

5. If of a right angle be the unit of measurement, express an angle which contains 22.5 degrees.

6. Show how to reduce English seconds to French seconds.

7. If the unit of measurement be 8°, what is the value of 10%.

8. If two of the angles of a triangle be expressed in grades, and the third in degrees, they are respectively as the numbers 5, 15, 18. Find the angles.

9. What is the value in degrees and grades of an angle which is the result of the revolution of a line 3} times round.

10. In what quadrants are the following angles found :145°, 969, 327°, 272, 272o.

11. If ao be taken as the unit of angular measurement, express an angle containing b. 12. What is the unit of measurement when a expresses

P

9 of a right angle?

CHAPTER II.

THE GONIOMETRIC FUNCTIONS.

S

B

4. It was formerly usual in works on Trigonometry to give the following definitions :

Let a circle be described from centre A, I with radius AB supposed to be unity, then

(1.) The sine of an arc BC is the perpendicular from one extremity, C, of the arc upon the diameter passing through the other extremity B.

Thus ČS is the SINE of the arc BC.

(2.) The cosine of an arc is the sine of the co ment of the arc. Thus, since DC is the complement of BC,

S'C is the COSINE of the arc BC. (3.) The tangent of an arc BC is a line

drawn from one extremity, B, of the arc touching the circle, and terminated in the diameter which passes through the other extremity, C, of the arc.

Thus, BT is the TANGENT of the arc BC. (4.) The cotangent of an arc is the tangent of the complement of Thus, DT' is the COTANGENT of the arc BC.

(5.) The secant of an arc BC is a line drawn from the centre through one extremity, C, of the arc, and terminated in the tangent at the other extremity.

Thus, AT is the SECANT of the arc BC. (6.) The cosecant of an arc is the secant of the complement of the arc. Thus, AT' is the COSECANT of the arc BC.

the arc.

the arc.

cosec A

(7.) The versed sine is that portion of the radius upon which the sine falls, which is included between the sine and the extremity of

Thus, SB is the VERSED SINE of the arc BC.

(8.) The coversed sine is the versed sine of the complement of the arc.

Thus, S'D is the COVERSED SINE of the arc BC. (9.) The suversed sine is the versed sine of the supplement of the arc. Thus, B'S is the SUVERSED SINE of the arc BC.

Representing the arc BC by A, it is usual to write the above functions thus :-Sin A, cos A, tan A, cot A, sec A, cosec A, vers A, covers A,. suvers A.

By mere inspection, the student will see that the following relations hold :

CS CS AD 1 (1.) Sin A = CS =

1 AS AT

AS AS AB 1 (2.) Cos A = S'C =

1 AC AT
BT
BT

AD 1 (3.) Tan A = BT

AB DT'

cot A' (4.) Sin? A + cos? A = CS? + S/C? = CS? + AS? = AC? = 1. (5.) Sec? A = AT? = AB + BT? = l + tan? A. (6.) Cosec? A = AT/2 = AD? + DT'2 1 + cot? A.

BT CS CS sin A (7.) Tan A

AB AS S'C (8.) Vers A = SB = AB AS = AB S'C = l - cos A. (9.) Covers A = S'D = AD AS' = AD CS = 1 sin A. (10.) Suvers A B'S = B'A + AS = l + cos A.

AS

sec A

S

=

BT -

cos A

It is more convenient, however, to define the sine, cosine, &c., as in the next article, according to which definitions they are commonly called TRIGONOMETRICAL RATIOS. The student will see that if the above definitions be so far modified that, instead of the lines them. selves, the goniometric functions be taken as the ratios which the lines respectively bear to the radius, they are included in the definitions of the next article.

Trigonometrical Ratios.

5. Let BAC be any angle, which we may denote by A, and P any point in the line AC. Draw PM perpendicular to AB.

Then (1.) Sin A

perpendicular

hyp.

PM
AP

A

M

-B
base AM
(2.) Cos A =

hyp. AP

perpendicular PM (3.) Tan A =

base

AM base

AM (4.) Cot A

perpendicular PM

hyp. AP
(5.) Sec A =

base AM
hyp.

AP
(6.) Cosec A
perpendicular

PM (7.) The versed sine is the remainder after subtracting the cosine from unity, orvers A

1 - cos A. (8.) The coversed sine is the remainder after subtracting the sine from unity, orcovers A

1 · sin A. (9.) The suvorsed sine is the sum of the cosine and unity, or

suvers A 1 + cos A. The last three are not much used in practice.

6. Comparing (1.) and (6.), (2.) and (5.), (3.) and (4.), of the last article, we see at once that the sine and cosecant, the cosine and secant, and the tangent and cotangent, are respectively each the reciprocal of the other.

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