Sine, cosine and tangent
At school we first learn how to measure angles and how to add and subtract angles. Later on comes the idea of sine, cosine and tangent. There is a question that is left unanswered until we learn the definitions of sine, cosine and tangent. The question is: we know that a ramp can be harder to walk on according to how much inclined it is, with flat being zero angle and 90° the hardest. Is there a relationship of how many units we walk forwards and how many units we go up if we are walking over a ramp with an angle greater than zero? This is exactly what sine, cosine and tangent are. Ratios that relate angles to sides of a triangle such that we have the answer for the previous mentioned question.
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AB = run = base BC = rise = height AC = ramp = hypotenuse |
In Euclidean geometry the sum of all internal angles must be equal to 180°. With the right triangle having one angle being 90°, the sum of the other two must be 90°. This explains why we never have to memorize rules or properties beyond 90°, because we don't need to.
[math]\displaystyle{ \sin(\alpha) = \frac{BC}{AC} }[/math]. It means how many units we are going up in respect to how many units we are walking forwards over the ramp. What happens if the angle is zero? Then we are not going up no matter how many units we go forwards. This explains [math]\displaystyle{ \sin(0^{\text{o}}) = 0 }[/math]. What happens if the angle is 90°? Then we have a 1:1 ratio, for each unit of the triangle's height we walk over the ramp by the same units. This explains [math]\displaystyle{ \sin(90^{\text{o}}) = 1 }[/math].
[math]\displaystyle{ \cos(\alpha) = \frac{AB}{AC} }[/math]. It means how many units we are going forwards in respect to how many units we are walking up over the ramp. What happens if the angle is zero? Then we have a 1:1 ratio, for each unit of the triangle's run we walk over the ramp by the same units. This explains [math]\displaystyle{ \cos(0^{\text{o}}) = 1 }[/math]. What happens if the angle is 90°? Then we are not moving forwards and going up. This explains [math]\displaystyle{ \cos(90^{\text{o}}) = 0 }[/math].
[math]\displaystyle{ \tan(\alpha) = \frac{BC}{AB} }[/math]. It means how many units we are going up in respect to how many units we are going forwards. What happens if the angle is 90°? The tangent doesn't exist. What happens if the angle is 45°? Then we have a 1:1 ratio, for each unit we go forwards we also go up by the same unit. This explains [math]\displaystyle{ \tan(45^{\text{o}}) = 1 }[/math]. What happens if the angle is zero? Then we are not going up for each unit we walk forwards. It's the same as being parallel.
Another way to interpret the ratios is a percentage. Sine, cosine and tangent assume values between 0 and 1, with the values between 0 and -1 being reflections. We can view that with vectors. Every time we decompose a vector in two parts, if we know the sine, cosine or tangent, we can find the proportion between each component. For example: in a Cartesian system, [math]\displaystyle{ \sin(90^{\text{o}}) = 1 }[/math] means that the vector has a zero horizontal component. [math]\displaystyle{ \tan(45^{\text{o}}) = 1 }[/math] means that both coordinates have the same value.
If you understand how to read the unit circle the trigonometric identities are all consequences of it. There are multiple formulas regarding the product, the quotient, addition and subtraction of angles. I have a textbook that states that as long as you understand the sum of angles, it's really all you need to know. I'm going with that textbook and going to show just two identities.
