## Differentiability in an Interval

• A function is said to be differentiable in (a, b) if it is differentiable at each and every point of the interval.
• Differentiable at [a, b] if

It is differentiable in (a, b)

f’ (a+) = limit(h→0) {(f(a+h)- f(a)) / h} = a finite quantity

f’ (a) = limit(h→0) {(f(b-h)- f(a)) /- h} = a finite quantity

• All trigonometric, logarithmic, exponential and polynomial functions are differentiable in their domain (Inverse Trigonometric Functions are not included).

Note-

• Derivative in an interval should be checked at all those points where f(x) may be discontinuous.
• For |f(x)| differentiability should be checked at those points where f(x) = 0.

For Example-

Consider a differentiable function f(x) whose graph is given as- Fig.- A

|f(x)| → Fig.- B

F(x) = 0 at A, B, C

At x= A |f(x)| is derivable; but may or may not be derivable at x = B & C.

Read More- Basic Meaning of Differentiations

Q-> Try to solve these functions?

(i) f(x) = |x3| (ii) f(x) = |x(x-1)|(iii) f(x) = (x-1)|x2 – 3x + 2|

### Important Points to Remember

• If f(x) and g(x) both are differentiable function at x = a then f(x) ± g(x), f(x)×g(x) and f(x)/g(x)[g(x)≠0] will also be differentiable at x = a.
• If f(x) differentiable and g(x) is not differentiable at x = a then f(x) ± g(x) will not be differentiable but nothing definite can be said about derivative of function f(x) × g(x) and f(x)/g(x)[where g(x) ≠ 0] at x = a.
• If f(x) and g(x) both are not differentiable at x = a then nothing definite can be said about derivative of function f(x) × g(x) and f(x)/g(x) [where g(x)≠0] at x = a.
• A derivative of a continuous function need not be continuous.

Ex- f(x)= Two cases

Case-1 f(x)= x2sin(1/x);  x≠0

Case-2 f(x)= 0;  x=0

• If f(x) is derivable at x = a and f(a)=0 and g(x) is continuous at x = a then product function f(x)g(x) will also be derivable at x =a.
• If f(x) is differentiable at x = a and p(h) and g(h) approaches to zero as h → 0

limit (h→0) f(a+p(h)) – f(a+ g(h))/ p(h)- g(h) = f’(a)

### Determination of functions which are differentiable and satisfying some given functional rule

Four Basic Steps

• Write the expression for f'(x) whether LHD or RHD

f'(x) = limit(h→0) {f(x+h) – f(x)}/ h

• Manipulate f(x+h)-f(x) using functional rule and get f'(x) in its simplifying form.
• Integrate both sides and f(x) with the constant of its integration.
• Using a suitable value of ‘x’, get the value of the constant of integration.

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