Differentiability at a point

Let f (x) be a real valued function defied on an open interval (a, b) and let c ϵ (a, b). Then f (x) is said to be differentiable or derivable at x = c, if   exist finitely.

This limit is called the derivative or differential coefficient of the function f (x) at x = c, and is denoted f’ (c) or Df (c) or {d/dx [f(x)]}ₓ _{= c}. Thus, f (x) is differentiable at x = c

The limits  is called the left hand derivative of f (x) at x = c and is denoted by f’ (c) or, Lf’ (c), while  is called the right hand derivative of f (x) at x = c and is denoted by f’ (c⁺) or Rf’(c).

Thus, f (x) is differentiable at x = c ⇒ Lf’ (c) = Rf’(c). If Lf’ (c) ≠ Rf’(c), we say that f (x) is not differentiable at x = c.

Meaning of differentiability of a function at a point: f (x) is differentiable at point P, iff there exists a unique tangent at point P. in order words, f (x) is differentiable at a point P iff the curve does not have P as a corner point.

Consider the function f (x) = |x|. This function is not differentiable at x = 0, because if we draw tangent at the origin as the limiting position of the chords on the left hand side of the origin, it is the line y = - x whereas the tangent at the origin as the limiting position of the chords on the right hand side of the origin is the line y = x. mathematically, left hand derivative at the origin is - 1 (slope of the line y = - x) and the fight hand derivate at the origin is 1 (slope of the line y = x).

Let f (x) be a differentiable function at a point P. then the curve y = f (x) has a unique tangent at P. since tangent at P is the limiting position of the chord PQ when Q → P. So, if f (x) is differentiable at a point P, then chords exist on both sides of P. consequently f (x) is continuous at P.

It follows from the above discussion that, if a function is not differentiable at x = c, then either it has (c, f (c)) as a corner point or it is discontinuous at x = c.

Also, every differentiable function is continuous.

Relation between continuity and differentiability: In the above discussion, we have observed that if a function is differentiable at a point, then it should be continuous at that point and a discontinuous function cannot be differentiable. This fact is provided in the following theorem.

If a function I differentiable at a point, it is necessarily continuous at that point. But the converse is not necessarily true.

Or

f (x) is differentiable at x = c ⇒ f (x) is continuous at x = c.

Converse: The converse of the above theorem is not necessarily true i.e. a function may be continuous at a point but may not be differentiable at that point because f (x) = |x| is continuous at x = 0 but it is not differentiable at x = 0.

Evaluate:  show that f (x) = |x| is not differentiable at x = 0.

Solution: we have,

(LHD at x = 0)

∴ (LHD at x = 0) ≠ (RHD at x = 0).

So, f (x) is not differentiable at x = 0.