a few last remarks:
Law of sines:
Take a triangle with base AB and vertex C, and side a opposite angle A, side b opposite angle B, and so on.
Assume angles A and B are both acute to make it easy. Now drop a perpendicular from vertex C to side AB hitting it at point Q between A and B. Let the length of that perpendicular CQ be x.
Now by the very definition of sin and cos, we have that sin(A) = x/b, and sin(B) = x/a. Hence x = b.sin(A) = a.sin(B), so sin(A)/a = sin(B)/b. voila! I don't use this much. It is also true that
sin(A)/a = sin(C)/c.Coordinate geometry and trig
There is a nice way to do geometry in the (x,y) plane, using coordinates that allows the algebraic tools of addition and multiplication to enhance the geometry. For each point (a1,a2) in the (x,y) plane, we imagine an arrow going from the origin (0,0) to the point (a1,a2). We call this arrow A. Let B be another arrow fro the origin (0,0) to the point (b1,b2). If we add the coordinates getting another point (a1+b1, a2+b2), we can ask how the arrow A+B from the origin (0,0) to (a1+b1, a2+b2) is related to the first two arrows. It turns out that the arrow A+B forms the diagonal of a parallelogram with sides A and B. The vertices of this parallelogram are the points (0,0), (a1,a2), (a1+b1, a2+b2), and (b1,b2).
E.g. if A = (1,0), and B = (0,1), then A+B = (1,1) is the diagonal of the square with vertices (0,0), (1,0), (1,1), and (0,1).
Consider the triangle with vertices (0,0), (a1,a2), and (a1+b1, a2+b2). The arrows A and A+B form two sides of this triangle with common vertex (0,0). The third side, which goes from the point (a1,a2) to the point (a1+b1, a2+b2), is parallel to the arrow B, and has the same length. Thus if the arrows A and B are perpendicular, the arrow A+B is the hypotenuse of a right triangle whose sides have the same lengths as the arrows A and B. If we denote length of an arrow by | |, then by Pythagoras we get |A+B|^2 = |A|^2 + |B|^2.
By the same reasoning, if we consider the triangle with two sides A and B, its third side is parallel to the arrow A-B, and has the same length. Hence by Pythagoras, if A and B are perpendicular, then |A-B|^2 = |A|^2 + |B|^2.
We will define a multiplication of arrows that captures this theorem and also the more general law of cosines.
To multiply two arrows A = (a1,a2) and B = (b1,b2), we define their “dot product”as A.B = a1b1 + a2b2, which is a number, rather than an arrow. It is easy to check that this multiplication has some of the properties of usual multiplication, like commutativity and distributivity for addition, and so on, but the product of two non - zero arrows can be zero. E.g. (1,0).(0,1) = 1.0 + 0.1 = 0+0 = 0.
Moreover, the product of an arrow with itself is exactly the square of its length, i.e. A.A = (a1)^2 + (a2)^2 = |A|^2, by Pythagoras.
In fact the dot product of two arrows is zero exactly when the arrows are perpendicular.
I.e. consider A and B as two sides of a triangle. Then the third side is parallel to the arrow A-B, and has the same length. Hence |A-B|^2 = (A-B).(A-B) = A.A – 2A.B + B.B =|A|^2 +|B|^2 – 2A.B. But if A and B are perpendicular, then by Pythagoras we must have |A-B|^2 = |A|^2 + |B|^2, so A.B must be zero.
If A and B are sides of any triangle with third side parallel to and of same length as A-B, then again |A-B|^2 = A|^2 +|B|^2 – 2A.B. This looks exactly like the law of cosines except that we have 2A.B in place of 2|A||B|cos© where c is the angle between A and B. Thus in fact A.B must equal |A||B|cos©.
If on the other hand we knew that A.B = |A||B|cos©, then we get the law of cosines by expanding the dot product |A-B|^2 = (A-B).(A-B) = A.A – 2A.B + B.B = |A|^2 +|B|^2 – 2A.B =|A|^2 +|B|^2 – 2|A||B|cos©.
This allows us to remember the easier formula A.B = |A||B|cos©, and then to recover the more complicated law of cosines.
It also gives a way to calculate cosines without a calculator.
E.g. the angle between the arrows A and B has cosine equal to (A.B)/|A||B|. E.g. the angle between A = (1,0) and B = (1,sqrt(3)) has cosine equal to 1/2. Remember what angle that is?To summarize that mess, define a product of two arrows A,B in the plane, beginning at the origin,
as: A.B = |A||B|cos©, where | | denotes length, and c is the angle between the two arrows.
Also define an addition of arrows, by placing the tail of one arrow at the head of the other and taking the sum to go from the tail of the first to the head of the second arrow.
Then the third side of the triangle with arrows A and B as two sides, is parallel to the arrow A-B, and of same length.
Then the law of cosines becomes the usual rule for expanding a square:
(A-B).(A-B) = A.A - 2A.B + B.B and since the angle between any vector and itself is zero, we get:
|A-B||A-B|cos(0) = |A||A|cos(0) - 2 |A||B| cos© + |B||B|cos(0).
Then since cos(0)m = 1, we get
|A-B|^2 = |A|^2 - 2|A||B| cos© + |B|^2, which is exactly the usual law of cosines.Caveat: I have not proved here that this multiplication is distributive, For that I probably need the law of cosines!
(sometime later)...
Well, duh, I guess that's the whole point: the law of cosines is equivalent to saying this multiplication is distributive, i.e. is a multiplication!
So this does not reprove the law of cosines, it just restates it in a more natural form.
Then the three term principle says that since the explicit multiplication rule A.B = a1b1 + a2b2, is also distributive, and agrees with the first one on vectors that are equal, it must agree as well on all vectors.My point here is to try to show how elementary math is illuminated when viewed from a higher point of view. No advanced math is being done here, but we are seeing elementary math more clearly I hope, by exposing its structure. This is what is not obtained from plug and chug treatments. If anyone has a book on trig, Saxon or otherwise, it might be interesting to compare its treatment with the one given here.By the way, if anyone has a child who looks at the law of cosines
|C|^2 = |A|^2 +|B|^2 – 2|A||B|cos©,
and remarks that the right hand side looks kind of like what you get when you expand (A-B)^2, or asks whether C = A-B, then
you have a potential mathematician - certainly a child who is thinking like one. Mathematics is about looking for patterns, and analogies.