Table of contents | |
Introduction to Trigonometry | |
Measurement of Angles | |
T-ratios (or Trigonometric Functions) | |
New Definition of T-ratios |
There are two systems of measurement of angles:
(i) Sexagesimal or English System: Here 1 right angle = 90 (degrees)
1 = 60’ (minutes)
1’ = 60" (seconds)
(ii) Circular System: Here an angle is measured in radians. One radian corresponds to the angle subtended by an arc of length ’r ’ at the centre of the circle of radius r. It is a constant quantity and does not depend upon the circle's radius.
(a) Relation between the two systems: Degree = Radian*180 / π
(b) If θ is the angle subtended at the centre of a circle of radius 'r',
by an arc of length 'l' then l / r = θ
Representation of Angle in Circular System
Note: l, r are in the same units and θ is always in radians.
Example.1. If the arcs of the same length in two circles subtend angles of 60° and 75° at their centres. Find the ratio of their radii.
Sol: Let r1 and r2 be the radii of the given circles and let their arcs of the same length s subtend angles of 60 and 75 at their centres.
Now,
⇒ 4r1 = 5r2 ⇒ r1 : r2 = 5 : 4
A Right Angled Triangle
In a right-angle triangle:
sin θ = p / h
cos θ = b / h
tan θ = p / b
cosec θ = h / p
sec θ = h / b
cot θ = b / p
'p' is perpendicular, 'b' is base and 'h' is hypotenuse.
Note : The quantity by which the cosine falls short of unity i.e. 1 - cos θ, is called the versed sine θ of θ and also by which the sine falls short of unity i.e. 1- sinθ is called the coversed sine of θ.
The three main trigonometric functions are then defined in terms of r and the coordinates x and y:
(The other functions are reciprocals of these)
This can give negative values of the trigonometric functions.
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2. What are T-ratios (Trigonometric Functions) in trigonometry? |
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