We’re all familiar with the concept of trigonometry. It’s most commonly used, of course, to determine the angles and lengths in right-angled triangles. Provided you know any three side lengths or angles (including the right angle) in a right-angled triangle, you can easily use sin, cos and tan to work out the sides and angles you don’t know.
You can also work out all the sides and angles in any triangle using the sine rule
and the cosine rule.
Alternatively, you can simply dissect any triangle into two right-angled triangles and use SOHCAHTOA from there.
However, the amount of people whose knowledge of trigonometry stops there surprises me. To many people, sin, cos and tan are just buttons on a calculator. The nature of simply plugging in values and receiving an irrational number is somewhat menial, particularly if you have little of an idea of what is happening. Sadly, this is what a lot of people have experienced to be trigonometry.
The reality is that the functions of sine, cosine and tangent are embedded in the foundations of modern mathematics and, as you’ll discover, the world around us. First, you must understand that each of these functions has its own graph.
These graphs act as a reference every time you use a trigonometric function. Whenever you type ‘sin(90)’ into your calculator, for example, the calculator will find 90° on the x-axis and return whatever y value the sine graph has at that point; hence why sin(90) = 1.
Now, that’s all very nice, but where exactly do these graphs come from? The answer may be a surprise to some people, considering that the trigonometric experiences of many focus around triangles. In fact, all of the trigonometric functions are created from circles.
The sine graph is created by plotting the angle of the radius of a circle against the y-coordinate. The cosine graph is created in exactly the same way, except the angle is plotted against the x-coordinate. If you’re confused, it makes a lot more sense visually.
You may have noticed that the cosine graph is exactly the same shape as the sine graph, except in the GIF it is rotated 90°. This is exactly the reason why, when the graphs are plotted on grids as above, the cosine graph is equivalent to the sine graph, omitting the fact that it is shifted, or translated, 90° to the left. Additionally, this is how the name ‘cosine’ appeared; the two coincide, they cohere, they coexist.
Tangent, however, is a different story. It is called tangent because the graph is created by drawing a circle adjacent to the y-axis, so that the axis is a tangent to the circle, and then plotting the points where the extended radius of the circle would touch the tangent. Once again, this is much more easily explained visually.
The characteristic that sets the tangent graph apart the most is that it includes asymptotes. An asymptote is the singularity on a graph that cannot possibly contain any values. On either side however, lines become exponentially closer to it but never actually touch it. This happens because as you can see in the GIF, when the radius is at 90° from its initial position (vertical, in other words), it is parallel to the tangent. If you were to scroll up and try to find the point of intersection between the tangent and the radius, you would be scrolling for a very long time; there is no point of intersection, hence why there are no values exactly on the asymptotes.
I understand that I’m not exactly a master of clarity, so I’ll try to explain in a more logical way why there is no tan(90).
Let’s go back to SOHCAHTOA. The last of the three formulae, you’ll recall, is:
To try out tan(90), we’ll have to make θ equal to 90°. That’s easy enough.
Now we’ll need to make this back into a triangle. The only thing we can do to do that is to make the adjacent side length equal 0.
This is now what our triangle looks like. Isn’t it wonderful?
Next, we need to input the values back into the SOHCAHTOA formula.
Oh dear. Dividing by zero never ends well, so let’s not attempt it. It appears that a value for tan(90) simply doesn’t exist.
You can see from the tangent graph that it’s not only at 90° that an asymptote appears. Each of the graphs is a repeating pattern, and the length of one cycle on a tangent graph is 180°. In other words, you have to travel 180° along the x-axis to get from a certain point in one pattern to the same point in the next.
This is why the asymptotes appear at ±90°, ±270°, ±540°, etc. The difference between each one is 180°.
So we’ve talked about the main three trigonometric functions and their respective graphs. You may have seen the graphs before, or you may not have. However, you’re less likely to know that these graphs are actually scattered around us in various aspects of everyday life.
Let’s start with a spring. Springs can be found in all manner of items, particularly in physical mechanisms. However, if you look at a spring directly from the side, this is what it will look like.
Oh look, a sine graph! What happens if we rotate our view 90° about the spring so that we’re looking at it from above?
A cosine graph appears. Not only is this a real-world visual representation of both graphs, but it also demonstrates that a cosine graph is simply a sine graph at a displacement of 90°. You can try it yourself.
As you’d expect, this can be observed in any helical object (meaning any object pertaining to the form of a helix). Common examples include drill bits, spiral staircases, and any object with a screw mechanism, such as a water bottle.
If you’re interested, the mathematical equations of the helix are as follows:
x = A cos t
This means that when a helix is observed from the x direction, it is simply a cosine graph that is vertically enlarged by A and horizontally enlarged by the reciprocal of t.
y = A sin t
This means that when the same helix is observed from the y direction, it is a sine graph enlarged to exactly the same scale as the cosine graph from the x direction.
z = b t
This just means that the helix has b length in the z direction. When viewed from the z direction, all you will see is a circle when radius of length A.
You can also map tides using a sine graph very simply. I’ve created one to show you now.
In this instance, we can say that the x-axis represents time of day in hours, and the y-axis represents height of tide in metres. Tides work on a 12-hour cycle, which you can see from the graph. In fact (disregarding any outliers which might affect the sea levels temporarily, such as a storm), tides will follow the exact pattern of a sine wave when plotted on a height-time graph.
Tangent graphs are also embedded in much of the real world, although they can’t exactly be observed in the same way that sine and cosine graphs can due to their asymptotes. However, their properties are often present.
Let’s revisit the spring. When a force is applied to the spring on one end, it is compressed, provided the force is in the direction of the spring. The more force is applied, the more compressed the spring becomes.
What happens if we draw a graph of the length of the spring against the force applied?
In this instance, the x-axis represents length of the spring in centimetres, and the y-axis represents force applied in Newtons.
On the graph, we get an asymptote, because you’d need an infinite amount of force to compress the spring so much that it had 0 length. It’s very similar to a tangent graph. In fact, the graph you’re seeing actually is a transformed tangent graph, with the equation y = tan(-x +90). The –x mirrors the graph horizontally (over the y-axis), while the +90 translates the graph 90° to the left so that the asymptote occurs when x = 0. It’ll become clearer when I show a wider perspective of the graph.
Another application of tangent graphs is to do with the motion of pendulums. Imagine a small ball hanging from the end of a string, which is attached to the roof of the interior of a car.
Once the car starts accelerating, the pendulum will swing in the opposite direction of motion. The equation that determines the angle between the pendulum and the vertical axis is:
Does this equation look familiar? That’s right, it’s the tangent formula from SOHCAHTOA! We’ve simply replaced ‘opposite’ with ‘ma’ and ‘adjacent’ with ‘mg’.
This equation also shows us why it’s impossible for the pendulum to be parallel to the roof (ignoring the fact that the ball is in the way). If the pendulum were parallel to the roof, then θ would be equal to 90°. As we know, there is no value for tan(90). To make it possible, either a would have to equal ∞, or m would have to equal either 0 or ∞, all of which are beyond the laws of physics. (Quantum theory may disagree, but we won’t get into that right now.)
Of course, you could just go into space, where g would equal 0, but then the formula wouldn’t apply there. You can’t cheat maths.
To finish, I’ll just leave this link here. It’s a brilliant interactive resource that lets you visualise the three basic trigonometric graphs (plus their hyperbolic counterparts) and how they are related to the circle. If you didn’t understand something in my explanations, then hopefully this will clear things up.
By the way, the scale is in radians as opposed to degrees. If you don’t know, radians are a way of expressing angles in a circle relative to π. If you delve further into them they become thoroughly interesting, but to understand the graph all you need to know is that π radians = 180°.
Thank you for reading.