Sine, Cosine and Tangent in Four Quadrants
Sine, Cosine and Tangent
The three main functions in trigonometry are Sine, Cosine and Tangent.
They are easy to calculate: The length of one side of a right angled triangle, |
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For a triangle with an angle θ, the functions are calculated this way:
Sine Function: |
sin(θ) = Opposite / Hypotenuse |
Cosine Function: |
cos(θ) = Adjacent / Hypotenuse |
Tangent Function: |
tan(θ) = Opposite / Adjacent |
Example: What is the sine of 35°?
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Using this triangle (lengths are only to one decimal place): sin(35°) = Opposite / Hypotenuse = 2.8/4.9 = 0.57... |
Cartesian Coordinates
Using Cartesian Coordinates you mark a point on a graph by how far along and how far up it is:
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| The point (12,5) is 12 units along, and 5 units up. |
Four QuadrantsWhen we include negative values, the x and y axes divide the space up into 4 pieces: Quadrants I, II, III and IV (They are numbered in a counter-clockwise direction) |
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In Quadrant I both x and y are positive, but ...
- in Quadrant II x is negative (y is still positive),
- in Quadrant III both x and y are negative, and
- in Quadrant IV x is positive again, while y is negative.
Like this:
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Example: The point "C" (-2,-1) is 2 units along in the negative direction, and 1 unit down (i.e. negative direction).
Both x and y are negative, so that point is in "Quadrant III"
Sine, Cosine and Tangent in the Four Quadrants
Now let us look at what happens when we place a 30° triangle in each of the 4 Quadrants.
In Quadrant I everything is normal, and Sine, Cosine and Tangent are all positive:
Example: The sine, cosine and tangent of 30°
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But in Quadrant II, the x direction is negative, and both cosine and tangent become negative:
Example: The sine, cosine and tangent of 150°
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In Quadrant III, sine and cosine are negative:
Example: The sine, cosine and tangent of 210°
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Note: Tangent is positive because dividing a negative by a negative gives a positive. |
In Quadrant IV, sine and tangent are negative:
Example: The sine, cosine and tangent of 330°
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There is a pattern! Look at when they are positive ...
- All three of them are positive in Quadrant I
- Sine only is positive in Quadrant II
- Tangent only is positive in Quadrant III
- Cosine only is positive in Quadrant IV
This can be shown even easier by:
Some people like to remember the four letters ASTC by one of these:
You can remember one of these, or maybe you could make up |
 This graph shows "ASTC" also. |
Two Values
Have a look at this graph of the Sine Function::
There are two angles (within the first 360°) that have the same value!
And this is also true for Cosine and Tangent.
The trouble is: Your calculator will only give you one of those values ...
... but you can use these rules to find the other value:
| First value | Second value | |
| Sine | θ | 180º - θ |
| Cosine | θ | 360º - θ |
| Tangent | θ | θ - 180º |
And if any angle is less than 0º, then add 360º.
We can now solve equations for angles between 0º and 360º (using Inverse Sine Cosine and Tangent)
Example: Solve sin θ = 0.5
We get the first solution from the calculator = sin-1(0.5) = 30º (it is in Quadrant I)
The other solution is 180º - 30º = 150º (Quadrant II)
Example: Solve tan θ = -1.3
We get the first solution from the calculator = tan-1(-1.3) = -52.4º
This is less than 0º, so we add 360º: -52.4º + 360º = 307.6º (Quadrant IV)
The other solution is 307.6º - 180º = 127.6º (Quadrant II)
Example: Solve cos θ = -0.85
We get the first solution from the calculator = cos-1(-0.85) = 148.2º (Quadrant II)
The other solution is 360º - 148.2º = 211.8º (Quadrant III)