While I was solving a problem something interesting came up if we know the dot product of 2 vectors, and one of the vectors is known. Can we find the other? The interesting bit about this is that even though the dot product behaves pretty much like normal algebraic multiplication 1, but that does not extend to properties like division for example if $a * b = c$ then $a = \frac{b}{c}$
Why is this so (other than the fact that we're talking about vectors)?
1 As Qiaochu Yuan points out that statement is simply inaccurate. Sorry.
The reason why the dot product of two vectors has no inverse is that it is not an injective function. In fact, if you fix a vector $v$ and a number $r$, then the set of all vectors $w$ that satisfy $v * w = r$ is a hyperplane of your vector space, and those usually contain more than one element.
In contrast to this, multiplication by a non-zero real number is an injective function (in fact, a bijective function), and so it has an inverse function, namely division by that number.
You should also note that multiplication of real numbers and dot products are fundamentally different in the sense that multiplication of two real numbers gives you back a real number, whereas the dot product of two vectors in general does not give you back a vector of the same space, but a real number (or an element of the field you're working over).