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5. The segment DA of the diameter passing through A, one extremity of
the arc AC, between the sine CD and the point A, is called the Versed sine of the arc AC, or of the angle ABC.
6. A straight line AE touching the circle at A, one extremity of the arc
AC, and meeting the diameter BC, which passes through the other
extremity, is called the Tangent of the arc AC, or of the angle ABC Cor. The tangent of half a right angle is equal to the radius. 7. The straight line BE, between the centre and the extremity of the tan
gent AE is called the Secant of the arc AC, or of the angle ABC. Cor. to Def. 4, 6, 7, the sine, tangent and secant of any angle ABC, are
likewise the sine, tangent, and secant of its supplement CBF. It is manifest, from Def. 4. that CD is the sine of the angle CBF. Let
CB be produced till it meet the circle again in I; and it is also manifest, that AE is the tangent, and BE the secant, of the angle ABI, or
CBF, from Def. 6. 7.
arc, which is the measure of any gi..
Let AC, MN be measures of the angle
ABC, according to Def. 1.; CD the B sine, DA the versed sine. AE the
OMD tangent, and BE the secant of the arc AC, according to Def. 4, 5, 6, 7, NO the sine, OM the versed sine, MP the tangent, and BP the secant of the arc MN, according to the same definitions. Since CD, NO, AE, MP are parallel, CD: NO :: rad. CB : rad. NB, and AE : MP :: rad. AB : rad. BM, also BE : BP :: AB: BM ; likewise because BC: BD :: BN : BO, that is, BA : BD :: BM : BO, by conversion and alternation, AD: MO :: AB : MB. Hence the corollary is manifest. And
therefore, if tables be constructed, exhibiting in numbers the sines, tangents, secants, and versed sines of certain angles to a given radius, they will exhibit the ratios of the sines, tangents, &c. of the same angles to
any radius whatsoever. In such tables, which are called Trigonometrical Tables, the radius is
either supposed 1, or some in the series 10, 100, 1000, &c. The use
and construction of these tables are about to be explained. 8. The difference between any angle
and a right angle, or between any arc and a quadrant, is called the Complement of that angle, or of that arc. Thus, if BH be perpendicular to AB, the angle CBH is the complement of the angle ABC, and the arc HC the complement of AC; also, the complement of the obtuse angle FBC is the angle HBC, its excess above a right angle; and the complement of the arc FC is HC.
9. The sine, tangent, or secant of the complement of any angle is called
the Cosine, Cotangent, or Cosecant of that angle. Thus, let CL or DB, which is equal to ČL, be the sine of the angle CBH; HK the tangent, and BK the secant of the same angle: CL or BD is the cosine, HK the cotangent, and BK the cosecant of the angle ABC.
Cor. 1. The radius is a mean proportional between the tangent and the
cotangent of any angle ABC ; that is, tan. ABC X cot. ABC=R2. For, since HK, BA are parallel, the angles HKB, ABC are equal, and
KHB, BAE are right angles; therefore the triangles BAE, KHB are
similar, and therefore AE is to AB, as BH or BA to HK. Cor. 2. The radius is a mean proportional between the cosine and se
cant of any angle ABC; or
cos. ABC X sec. ABC=R2. Since CD, AE are parallel, BD is to BC or BA, as BA to BE.
In a right angled plane triangle, as the hypotenuse to either of the sides, so
the radius to the sine of the angle opposite to that side ; and as either of the sides is to the other side, so is the radius to the tangent of the angle opposite to that side.
Let ABC be a right angled plane triangle, of which BC is the hypotenuse. From the centre C, with any radius CD, describe the arc DE; draw DF at right angles to CE, and from E draw EG touching the circle in E, and meeting CB in G; DF is the sine, and EG the tangent of the arc DE, or of the angle C.
The two triangles DFC, BAC, are equiangular, because the angles DFC, BAC are right angles, and the
B angle at C is common. Therefore, 1 CB : BA :: CD : DF; but CD is
the radius, and DF the sine of the angle C, (Def. 4.); therefore CB : BA :: R: sin. C.
Also, because EG touches the circle in E, CEG is a right angle, and therefore equal to the angle BAC; and since the angle at C is common
. F E A to the triangles CBA, CGE, these triangles are equiangular, wherefore CA : AB :: CE: EG; but CE is the radius, and EG the tangent of the angle C; therefore, CA : AB :: R : tan. C.
Cor. 1. As the radius to the secant of the angle C, so is the side adja- , cent to that angle to the hypotenuse. For CG is the secant of the angle G (def. 7.), and the triangles CGE, CBA being equiangular, CA : CB :: CE : CG, that is, CA : CB :: R: sec. C.
Cor. 2. If the analogies in this proposition, and in the above corollary be arithmetically expressed, making the radius = 1, they give sin. C= AB on ABC BCA C=
F. Also, since sin. C=cos. B, because B
is the complement of C, cos. B=ab, and for the same reason, cos. C= AC BC
Cor. 3. In every triangle, if a perpendicular be drawn from any of the angles on the opposite side, the segments of that side are to one another as the tangents of the parts into which the opposite angle is divided by the perpendicular. For, it in the triangle ABC, AD be drawn perpendicular to the base BC, each of the triangles CAD, ABD being right angled, AD: DC ::R : tan. CAD, and AD: DB :: R : tan. DAB; therefore, ex B æquo, DC : DB : : tan. CAD : tan. BAD.
SCHOLIUM. The proposition, just demonstrated, is most easily remembered, by stating it thus : If in a right angled triangle the hypotenuse be made the radius, the sides become the sines of the opposite angles; and if one of the sides be made the radius, the other side becomes the tangent of the opposite angle, and the hypotenuse the secant of it.
PROP. II. THEOR.
The sides of a plane triangle are to one another as the sines of the opposite
angles. From A any angle in the triangle ABC, let AD be drawn perpendicular to BC. And because the triangle ABD is right angled at D, AB : AD:: R: sin. B; and for the same reason, AC : AD :: R: sin. C, and inversely, AD : AC :: sin. C:R; therefore, ex æquo inversely, AB : AC :: sin. C : sin. B. In the same manner it may be demonstrated, that AB : BC :: sin. C : sin. A.
PROP. III. THEOR.
The sum of the sines of any two arcs of a circle, is to the difference of their
sines, as the tangent of half the sum of the arcs to the tangent of half their difference.
Let AB, AC be two arcs of a circle ABCD; let E be the centre, and AEG the diameter which passes through A ; sin. AC+sin. AB : sin. AC -sin. AB :: tan. (AC+AB): tan. 3 (AC-AB).
Draw BF parallel to AG, meeting the circle again in F. Draw BH and CL perpendicular to AE, and they will be the sines of the arcs AB and AC; produce CL till it meet the circle again in D; join DF, FC, DE, EB, EC, DB.
Now, since EL from the centre is perpendicular to CD, it bisects the line CD in L and the arc CAD in A: DL is therefore equal to LC, or to the sine of the arc AC; and BH or LK being the sine of AB, DK is the sum of the sines of the arcs AC and AB, and CK is the difference of their sines; DAB also is the sum of the arcs AC and AB, because AD is equal to AC, and BC is their difference. Now, in the triangle DFC, because FK is perpendicular to DC, (3. cor. 1.), DK : KC :: tan. DFK : tan. CFK; but tan. DFK=tan. } arc. BD, because the angle DFK (20. 3.) is the half of DEB, and therefore measured by half the arc DB. For the same reason, tan. CFK=tan. } arc. BC; and consequently, DK : KC :: tan. } arc. BD : tan. į arc. BC. But DK is the sum of the sines of the arcs AB and AC; and KC is the difference of the sines ; also BD is the sum of the arcs AB and AC, and BC the difference of those arcs