Inverse Sine, Cosine, Tangent

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

And they are very similar functions ... so we will look at the Sine Function and then Inverse Sine to learn what it is all about.

Sine Function

The Sine of angle θ is:

  • the length of the side Opposite angle θ
  • divided by the length of the Hypotenuse

Or more simply:

sin(θ) = Opposite / Hypotenuse

triangle showing Opposite, Adjacent and Hypotenuse

Example: What is the sine of 35°?

Using this triangle (lengths are only to one decimal place):

sin(35°) = Opposite / Hypotenuse = 2.8/4.9 = 0.57...

The Sine Function can help us solve things like this:

 

Example: Use the sine function to find "d"

We know

* The angle the cable makes with the seabed is 39°

* The cable's length is 30 m.

And we want to know "d" (the distance down).

 

 

Start with:   sin 39° = opposite/hypotenuse = d/30
Swap Sides:   d/30 = sin 39°
Use a calculator to find sin 39°:   d/30 = 0.6293…
Multiply both sides by 30:   d = 0.6293… x 30 = 18.88 to 2 decimal places.

The depth "d" is 18.88 m

Inverse Sine

But what if it is the angle we don't know?

This is where "Inverse Sine" comes in.

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

The symbol for inverse sine is sin-1

 

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"

 

 

Start with:   sin a° = opposite/hypotenuse = 18.88/30
Calculate 18.88/30:   sin a° = 0.6293...
     
What angle has sine equal to 0.6293...?
The Inverse Sine will tell us.
     
Inverse Sine:   a° = sin-1(0.6293...)
     
Use a calculator to find sin-1(0.6293...):   a° = 39.0° (to 1 decimal place)

The angle "a" is 39.0°

They Are Like Forward and Backwards!

sin vs sin-1
  • The Sine function sin takes an angle and gives us the ratio “opposite/hypotenuse”
  • Inverse Sine sin-1 takes the ratio “opposite/hypotenuse” and gives us the angle.

Example:

Sine Function:   sin(30°) = 0.5          
Inverse Sine:             sin-1(0.5) = 30°

Calculator

calculator-sin-cos-tan On the calculator you would 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 to 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

Right-Angled Triangle

 

What About "cos" and "tan" ... ?

Exactly the same idea.

The Cosine of angle θ is:

cos(θ) = Adjacent / Hypotenuse

And Inverse Cosine is :

cos-1 (Adjacent / Hypotenuse) = θ

Right-Angled Triangle

 

Example: Find the size 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)

 

 

The Tangent of angle θ is:

tan(θ) = Opposite / Adjacent

So Inverse Tangent is :

tan-1 (Opposite / Adjacent) = θ

Right-Angled Triangle

 

Example: Find the size 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.

So sin-1(y) can be written arcsin(y),
and cos-1(y) is also arccos(y)
and tan-1(y) is also arctan(y)

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 and Cosine do ...

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 and bottom? (I have showed it as dots, but it's not really part of it).

Because to be a function it must 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, and 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.

So, by chopping it off like that we get just one answer, and you are supposed to know there could be other answers.

Tangent and Inverse Tangent

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

Tangent
 
Inverse Tangent