Beg pardon?

Like a lot of people, I hated maths at school. Like most subjects, it was taught badly with the only supposed "motivation" to learn being punishment if you didn't. It took a magical American book, bought when we spent months there when I was 10, to turn things around, but by the time I was settled back in school, it was really too late to learn as much as I'd realised I wanted from maths. A subject I eventually loved, but alas, left school a year later, and of course know nothing now.

Well, here's Brian Cox telling us something tonight. It is completely, totally, incomprehensible to me. He just reduced delta x, but I'm buggered if I know what that means.