So y(2+△x)=(2+△x)? -(2+△x)

=2+△x +△x*△x

Therefore, y (2+ 1) = 4, y (2+0.1) = 2.1,y (2+0.01) = 2.010.

And dy = y'* dx, so dy(x=2)=3dx.

Dy is differential and δ y is increment of function.

When the function is differentiable, δy =δx+a(x), where a is constant (tangent slope of the function at this point) and a(x) is δ x->; 0 is an infinitesimal quantity greater than Δ x,

Difference dy = Aδx = A dx.