The principle of local linearity tells us that if we zoom in on a point where a function y = f (x) is differentiable, the function should become indistinguishable from its tangent line. That is, a dif...The principle of local linearity tells us that if we zoom in on a point where a function y = f (x) is differentiable, the function should become indistinguishable from its tangent line. That is, a differentiable function looks linear when viewed up close.