Get ready for the revolutionary and awe-inspiring explanation to trigonometric ratios. This is the only place in the world to provide such insights and revelations.

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We have learned

• Many applications are mathematically modeled using the sides of right angled triangles.

• the set of similar right angled triangles are defined by an angle.

What parameter of triangles are used in applications?

- measure of angles
- length of sides
- both the above
- both the above

The answer is 'both the above'. Though the set of similar right angled triangles is specified by an angle, to narrow down to a specific right angled triangle, one of the sides is specified.

A right angled triangle is specified by

• an angle - that specifies the set of similar right angled triangles

• length of one of the three sides of the triangle

When using this, it may be required to find the length of other sides of the triangle as the specified side may not be the one required in the calculations.

Angle defines set of similar right angled triangles. Providing one of the sides narrows it down to a specific right angled triangle.

**Specifying Right Angled Triangle: ** Set of similar right angled triangles are defined by one parameter: "angle". And to narrow down to one specific triangle, one side is additionally specified.

In a right angled triangle, When an angle and one of the sides is specified, what does one require to compute the other sides of the triangle? `a` and `theta` are given. Need to compute `b` and `c`.

- Other sides cannot be computed
- Given one angle, the ratio of two sides is a constant
- Given one angle, the ratio of two sides is a constant

The answer is 'Given one angle, the ratio of two sides is a constant'

In the figure

• `a/c = m/o = p/r =` constant

• `b/c = n/o = q/r =` a different constant

• `a/b = m/n = p/q =` another constant

When specifying the ratio of the sides, two sides out of the three sides of the triangles are used. It can either be `a/c`, or `b/c` or `a/b`. How do you identify which sides are used in a given ratio?

- Give the sides specific names with respect to the specified angle
- Give the sides specific names with respect to a the right angle
- both the above
- both the above

The answer is 'both the above'

The sides are called

• hypotenuse in reference to the right-angle,

• opposite side in reference to the given angle, and

• adjacent side in reference to the given angle.

Given set of right angled triangles defined by an angle `theta`.

• position of right angle is chosen.

• the position of the given angle is chosen • the side opposite to the right angle is fixed and called "hypotenuse"

• the side opposite to the given angle is fixed and called "opposite side"

• the side adjacent to the given angle is fixed and called "adjacent side"

If the given angle is chosen differently... • the side opposite to the right angle is "hypotenuse"

• the side opposite to the given angle is "opposite side"

• the side adjacent to the given angle is "adjacent side"

In a right angled triangle, When an angle and one of the sides is specified, the ratio of length of sides is constant.

• Ratio between opposite side and hypotenuse is required if hypotenuse is given and opposite side is to be computed (or vise versa).

• Ratio between adjacent side and hypotenuse is required if hypotenuse is given and adjacent side is to be computed (or vise versa).

• Ratio between opposite side and adjacent side is required if opposite side is given and adjacent side is to be computed (or vice versa).

How do we refer to which ratio we are using in the calculations?

- Give each of these a name and refer by the name.
- Give each of these a name and refer by the name.
- Describe each whenever the ratio is to be used.

The answer is 'Give each of these a name and refer by the name.'

For a set of similar right angled triangle specified by angle `theta`, the following ratios are defined. These are some of the trigonometric ratios Trigonometric Ratios:

`sin theta = text(opposite)/text(hypotenuse)`

`cos theta = text(adjacent)/text(hypotenuse)`

`tan theta = text(opposite)/text(adjacent)`

In the next set of pages, why the ratios are named as `sin`, `cos`, and `tan` is explained. Pl. continue...

What does 'trigonometry' mean?

- It is just a name.
- `color(coral)(text(trigon)) + color(deepskyblue)(text(metry)),``text( means ) color(coral)(text(triangle )) + color(deepskyblue)(text(measurement procedures))`
- `color(coral)(text(trigon)) + color(deepskyblue)(text(metry)),``text( means ) color(coral)(text(triangle )) + color(deepskyblue)(text(measurement procedures))`

Answer is '`color(coral)(text(trigon)) + color(deepskyblue)(text(metry)),``text( means ) color(coral)(text(triangle )) + color(deepskyblue)(text( measurement procedures))`'

Trigonometric Ratios for an angle are defined as ratio of sides of Right Angled Triangle.

**Trigonometric Ratios: **

`sin theta = text(opposite)/text(hypotenuse)`

`cos theta = text(adjacent)/text(hypotenuse)`

`tan theta = text(opposite)/text(adjacent)`

*Solved Exercise Problem: *

Say `sin` to practice the pronunciation.

- Pronunciation : Say the answer once

Spelling: Write the answer once

The `sin` is pronounced as sine.

*Solved Exercise Problem: *

Say `cos` to practice the pronunciation.

- Pronunciation : Say the answer once

Spelling: Write the answer once

The `cos` is pronounced as cos.

*Solved Exercise Problem: *

Say `tan` to practice the pronunciation.

- Pronunciation : Say the answer once

Spelling: Write the answer once

The `tan` is pronounced as tan.

Note that when `sin A` is written, it does not mean four variables `s`, `i`, `n` and `A`. The same can be explained for `cos A` and `tan A`

`sin A` is a function of variable `A`.

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