Q. 2 F5.0( 1 Vote )

Verify Rolle’s theorem for each of the following functions on the indicated intervals :

f(x) = x(x – 4)2 on [0, 4]

Answer :

First let us write the conditions for the applicability of Rolle’s theorem:


For a Real valued function ‘f’:


a) The function ‘f’ needs to be continuous in the closed interval [a,b].


b) The function ‘f’ needs differentiable on the open interval (a,b).


c) f(a) = f(b)


Then there exists at least one c in the open interval (a,b) such that f’(c) = 0.


Given function is:


f(x) = x(x – 4)2 on [0,4]


Since, given function f is a polynomial it is continuous and differentiable everywhere i.e., on R.


Let us find the values at extremums:


f(0) = 0(0 – 4)2


f(0) = 0


f(4) = 4(4 – 4)2


f(4) = 4(0)2


f(4) = 0


f(0) = f(4), Rolle’s theorem applicable for function ‘f’ on [0,4].


Let’s find the derivative of f(x):



Differentiating using UV rule,



f’(x) = ((x – 4)2×1) + (x×2×(x – 4))


f’(x) = (x – 4)2 + 2(x2 – 4x)


f’(x) = x2 – 8x + 16 + 2x2 – 8x


f’(x) = 3x2 – 16x + 16


We have f’(c) = 0 cϵ(0,4), from the definition given above.


f’(c) = 0


3c2 – 16c + 16 = 0






ϵ(0,4)


Rolle’s theorem is verified.


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