The **sine** function sin takes angle θ and gives the ratio *opposite* **hypotenuse **

The **inverse sine** function sin-1 takes the ratio *opposite***hypotenuse ** & gives angleθ

And cosine and tangent follow a similar idea.

### Example (lengths are only to one decimal place):

### And now for the details:

Sine, Cosine & Tangent are all based on a Right-Angled Triangle

They are very similar functions ... So we will look at the **Sine Function** và then **Inverse Sine** to lớn learn what it is all about.

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## Sine Function

TheSineofangle**θ**is:

**length of the side Opposite**angle

**θ**divided by the

**length of the Hypotenuse**

Or more simply:

sin(θ) = Opposite / Hypotenuse

### Example: What is the sine of 35°?

Using this triangle (lengths are only lớn one decimal place): sin(35°) = Opposite / Hypotenuse |

### Example: Use the **sine function** lớn find **"d"**

We know

The angle the cable makes with the seabed is 39° The cable"s length is 30 m.and we want lớn know "d" (the distance down).

## Inverse Sine Function

But sometimes it is the **angle** we need to find.

This is where "Inverse Sine" comes in.

It answers the question "what **angle** has sine equal lớn opposite/hypotenuse?"

The symbol for inverse sine is **sin-1**, or sometimes **arcsin**.

### Example: Find the angle **"a"**

We know

The distance down is 18.88 m.The cable"s length is 30 m.and we want to know the angle "a"

sin takes an

**angle**và gives us the

**ratio**"opposite/hypotenuse"sin-1 takes the

**ratio**"opposite/hypotenuse" and gives us the

**angle.**

## Calculator

On the calculator you press one of the following (depending on your brand of calculator):either "2ndF sin" or "shift sin". |

On your calculator, try using sin and then sin-1 khổng lồ see what happens

## More Than One Angle!

Inverse Sine **only shows you one angle** ... But there are more angles that could work.

### Example: Here are two angles where opposite/hypotenuse = 0.5

**In fact there are infinitely many angles**, because you can keep adding (or subtracting) 360°:

Remember this, because there are times when you actually need one of the other angles!

## Summary

The Sine of angle **θ** is:

sin(θ) = Opposite / Hypotenuse

And Inverse Sine is :

sin-1 (Opposite / Hypotenuse) = θ

## What About "cos" và "tan" ... ?

Exactly the same idea, but different side ratios.

CosineThe Cosine of angle **θ** is:

cos(θ) = Adjacent / Hypotenuse

And Inverse Cosine is :

cos-1 (Adjacent / Hypotenuse) = θ

### Example: Find the kích thước of angle a°

cos a° = Adjacent / Hypotenuse

cos a° = 6,750/8,100 = 0.8333...

a° = **cos-1** (0.8333...) = **33.6°** (to 1 decimal place)

Tangent

The Tangent of angle **θ** is:

tan(θ) = Opposite / Adjacent

So Inverse Tangent is :

tan-1 (Opposite / Adjacent) = θ

### Example: Find the kích cỡ of angle x°

tan x° = Opposite / Adjacent

tan x° = 300/400 = 0.75

x° = **tan-1** (0.75) = **36.9°** (correct to 1 decimal place)

## Other Names

Sometimes sin-1 is called **asin** or **arcsin****Likewise cos-1 is called acos** or **arccos****And tan-1 is called atan** or **arctan**

### Examples:

**arcsin(y)**is the same as

**sin-1(y)**

**atan(θ)**is the same as

**tan-1(θ)**

**etc.**

## The Graphs

And lastly, here are the graphs of Sine, Inverse Sine, Cosine and Inverse Cosine:

Sine

Inverse Sine

Cosine

Inverse Cosine

Did you notice anything about the graphs?

They look similar somehow, right?But the Inverse Sine and Inverse Cosine don"t "go on forever" like Sine & Cosine vày ...Let us look at the example of Cosine.

**Here is Cosine** and **Inverse Cosine** plotted on the same graph:

**Cosine and Inverse Cosine**

They are mirror images (about the diagonal)

But why does Inverse Cosine get chopped off at top & bottom (the dots are not really part of the function) ... ?

**Because to be a function it can only give one answer** **when we ask "what is cos-1(x) ?" **

**One Answer or Infinitely Many Answers**

**But we saw earlier that there are infinitely many answers**, và the dotted line on the graph shows this.

So yes there **are** infinitely many answers ...

... But imagine you type 0.5 into your calculator, press cos-1 and it gives you a never ending list of possible answers ...

So we have this rule that **a function can only give one answer**.

Xem thêm: Soạn Văn Bài: Tổng Kết Về Từ Vựng Luyện Tập Tổng Hợp Lớp 9, Soạn Bài Tổng Kết Về Từ Vựng (Luyện Tập Tổng Hợp)

So, by chopping it off lượt thích that we get just one answer, but **we should remember that there could be other answers**.

## Tangent & Inverse Tangent

And here is the tangent function và inverse tangent. Can you see how they are mirror images (about the diagonal) ...?

Tangent

Inverse Tangent

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