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Exercise 2.3 · Q55

Q.Rolle's theorem holds for the function f(x)=(x−2)log⁡x, x∈[1,2]f(x) = (x - 2)\log x,\ x \in [1, 2], show that the equation xlog⁡x=2−xx\log x = 2 - x is satisfied by at least one value of xx in (1,2)(1, 2).

Maharashtra MsbshseTextbookSubjectiveImportance★★★★★
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f(x)=(x−2)log⁡xf(x)=(x-2)\log x is continuous on [1,2][1,2] and differentiable on (1,2)(1,2) (product of continuous/differentiable functions, and log⁡x\log x is defined and smooth for x>0x>0).

f(1)=(1−2)log⁡1=(−1)(0)=0f(1)=(1-2)\log1=(-1)(0)=0. f(2)=(2−2)log⁡2=0(log⁡2)=0f(2)=(2-2)\log2=0(\log2)=0. So f(1)=f(2)=0f(1)=f(2)=0 — all hypotheses of Rolle's theorem hold, so there exists c∈(1,2)c\in(1,2) with f′(c)=0f'(c)=0.

f′(x)=log⁡x+(x−2)⋅1x=log⁡x+1−2xf'(x)=\log x+(x-2)\cdot\dfrac1x=\log x+1-\dfrac2x. …

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