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Differentiation from first principles, animated

A five-scene guided mini-lesson on y = x²: secant → tangent → the limit → f′(x) = 2x. Next / Back drives the story (← → keys work too).
gradient of PQ  =  f(1+h) − f(1)h  =  (1+h)² − 1²h
1 + 2h + h² − 1h  =  2h + h²h
h (2 + h)h  =  2 + h(legal — h ≠ 0)
as h → 0:  gradient → 2 + 0 = 2— the tangent gradient we watched