In this notes we'll cover topics about Trigonometry. We'll study about what is Trigonometry, why one should study Trigonometry and then about the basic formulae of Trigonometric Ratios.
Trigonometry is the study of triangles. More specifically right angled triangles.
To understand why one should use Trigonometry, it is important to understand where you can use it or apply it. Let us imagine that you are visiting the Statue of Unity (world's tallest Statue, situated in Gujarat-India).
You want to know the height of this statue by standing at a distance from it without actually measuring the height of it. We can visualize a right angled triangle being formed by a ray of sight from your eye to the top of the statue (this forms the hypotenuse), the distance between you and the statue (as one side of the triangle) and the line representing the height of the statue (as the third side of the triangle). This visualization is shown in the image below.
Applying some basic trigonometry formulas it becomes easy to calculate the height of the statue.
What is Trigonometry?
Trigonometry consists of three words - tri, gon and metron. Tri means three, gon means sides and metorn means measure. Hence we can say that Trigonometry is the study of relationships between the sides and angles of a triangle, more specifically right angle triangle.
Consider a right angle triangle ABC, where as shown in the figure below
Proof: and are acute angle triangles
We know that the sum of all the angles in a triangle is . Hence in our example of we can say that,
In , is , so we can solve the above statement as:
Since the addition of and is equal to . Both these angles are less than . Therefore, and are acute angles.
What are acute angles?
If the measure of an angle is less than , that angle is known as an acute angle.
Let us start with , as shown in the figure
Remember: Hypotenuse is the longest side of a right angle triangle.
Similarly for , as shown in the figure
Remember: The terminology for Trigonometry changes with respect to angles.
Now let us learn about the different trigonometric ratios with their abbreviations. For explanation purpose let's consider these ratios with respect to
Note: "co" before the ratios stands for the Complementary of that specific ratio.
Deriving the values of sine and co-sine
Let us now derive the values of and based on the trigonometric ratios that we learned above
Formulae for :
Formulae for :
Using these formulas:
Remember: and are fundamental trigonometric ratios and the others are derived ratios.
We have now used all the sides of triangle ABC to derive the values of and . In a similar manner, you can now derive the and for .
Deriving the values of other trigonometric ratios from sine and co-sine
### Secant of $\angle A$
### Co-Secant of $\angle A$
Since you now know how to calculate the trigonometric ratios, here's a tip for remembering these formulae:
We have and as the fundamental trigonometric ratios
If you know the formulae of and you can easily derive others
tan is the ratio of and and is the reciprocal of
is the reciprocal of
And is the reciprocal of