I am asking for books/any other online sources with the following philosphy$^{[1]}$:
This is a mathematics, not a programming book. It's intended for students, mathematics teachers and mathematicians who are just starting to explore mathematics on their own computers. In studying it, and especially working through the problems, readers will get to know many new elementary topics and learn as much from the extensive exercises as from the examples
Which basically means you have some problems, and with your computers you find and discover new patterns, which are tedious to do by hand, and then you prove them without computer.
Notes :
$^{[1]}$ The only book I know is Arthur Engel's "Exploring mathematics with your computer" , but it's not available at a reasonable price. The text is copied from it's preface, from the preview at Amazon. I forgot to mention, but I know Project Euler too, and it moderately fits to my criterion.
$[2]$ Related to my this question .
$[3]$ Books like "A=B" doesn't quite fits to my criterion, because you don't prove stuff with hypergeometric series there, the computer proves it for you. But "Computational recreations in mathematica" (by Illan Vardi) fits to my criterion, because you prove the stuff yourself.