Wikipedia. Learning online and from books I'm an high school student who likes mathematics.
So my question is the following.
At this level of math is Wikipedia a good resource for expanding what I learn (e.g., I learn about something but I need to dig deeper)?
At school for example I find these articles very interesting.
Trigonometric identities.
Complex number
Binomial theorem
and so on...
But I also like looking at some pieces of mathematics that aren't taught at school, like
Fibonacci number
and other articles...
I also, of course, explore this site as well some youtube videos related to math (ex Numberphile on youtube)
So, let me reformulate the initial question.
Is this "method" for learning math ok?  I know that Wikipedia is not like
a book for learning but quite often it has taught me something in a solid way.
Also do you know other online resources, as well as books?
ps I'm not English (I'm Italian) so I made some errors
 A: The best method for learning at your level is to spend time thinking. While this sounds quite broad, it is the best advice that can be given. Let me elaborate. Say you are reading your textbook on quadratics. The author(s) defines a quadratic equation to be an equation of the form $$y = ax^2 + bx +c.$$ So instead of simply doing questions that test your computation ability, why don't you investigate what properties the quadratic has? For example, ask yourself, how many roots can an equation of this form have? You will find that it will have at most two (This is the fundamental theorem of algebra). Then you may proceed to ask, under what conditions do you have only one root, only complex roots, etc. 
Then you may ask, how do I determine the gradient of such a function? This will lead to concepts of differentiation and calculus. Wikipedia is a good resource, but it should be used as more of a thought provoking resource.
EDIT: Also, a great thing I like to show kids that are in high school is something called modular arithmetic. You may enjoy this, https://en.wikipedia.org/wiki/Multiplicative_group_of_integers_modulo_n 
