# What was Weierstraß‘s geometric formulation of derivatives?

I was reading a discussion on $$\epsilon-\delta$$ definition of limits, continuity and derivatives, and there someone wrote this line $$f(x_0 +v) = f(x_0) + J(v)+ r(v), ~~ r(v)= \omega (v)$$

I’m looking for source of the above equation, and someone told me it is related to Weierstraß’s formulation of derivatives. Can someone recommend me a proper guide (in English) for Weierstraß’s theory of tangents and secants.