By differential coefficient, we mean the derivative of a continuous function. This represents the limit of the ratio of increment in the dependent variable; say ϒ corresponding to a small increment in the independent variable say, x as the latter tends to zero.

To make the point more clear, let y = f(x), Let us suppose that when x is increased by a small increment δx, then ϒ increase by a small increment δY. In such a case, we have

Y + δy = f(x + δx)

Thus, the change in the value of the function is,

(Y + δY) – Y = f(x+δx ) – f(x)

= δY = f(x +x ) – f(x)

= δY/δx = f(x +h)-f(x)/h

Where δY/δx is the incremental ratio of the dependent variable y with respect to x, the independent variable. When δx→ 0, and *the limiting value of f(f(x+δx)-f(x)/δx** i.e. ** δY/δx exists, then we say that y is differentiable with respect of y with respect to x.*

However, as a matter of convenience if is represented by

dy/dx=lim f(x+h)-f(x)/h

h→ 0