There are several ways of getting bold mathematical symbols. A classic way is directly provided by LaTeX. Take a look at this code:
\boldmath $y=f(x)$\unboldmath
It works in the following way:
In text mode, we switch to bold math alphabets.
We enter the math mode, in which bold symbols are always chosen, if available.
We leave the math mode.
While in the text mode, we switch the math alphabets back to normal—nonbold.
But that's for making all symbols of a formula bold. Such kind of emphasizing is rather rare today, as it destroys the uniform grayness of the text from a typographer's point of view.
A more common requirement is to get bold versions of certain symbols. For example, bold symbols are often used for vectors and number systems.
In this recipe, we will take the most recommended approach to get bold symbols.