Calculus Made Easy
Chapter I
A little bit of. The sum of.
These dreadful symbols are:
(1) d which merely means “a little bit of.” Thus dx means a little bit of x; or du means a little bit of u. But you will find that these little bits (or elements) may be considered to be indefinitely small.
(2) ∫ which is merely a long S, and may be called (if you like) “the sum of.” Thus ∫dx means the sum of all the little bits of x; or ∫dt means the sum of all the little bits of t.
The word “integral” simply means “the whole.” If you think of the duration of time for one hour, you may (if you like) think of it as cut up into 3600 little bits called seconds. The whole of the 3600 little bits added up together make one hour.
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