I was learning calculus and it seemed very interesting than Olympiad topics like Algebra, NT, Geometry and Combinatorics.
But I was struck here. I used this relation many times. But I don't know the proof for this. I hovered internet for over four days continuously but can't find it anywhere? Please help me to know the proof for this relation.
Please please make me know about this proof. Atleast please send me the link where I can get the proof for this.