Maths-
General
Easy

Question

Let f left parenthesis x right parenthesis and g left parenthesis x right parenthesis are defined and differentiable for x greater or equal than x subscript 0 end subscript and f open parentheses x subscript 0 end subscript close parentheses equals g open parentheses x subscript 0 end subscript close parentheses comma f to the power of ´ end exponent open parentheses x close parentheses greater than g ´ left parenthesis x right parenthesis for x greater than x subscript 0 end subscript comma then

  1. f open parentheses x close parentheses less than g left parenthesis x right parenthesis for some x greater than x subscript 0 end subscript  
  2. f open parentheses x close parentheses equals g left parenthesis x right parenthesis for some x greater than x subscript 0 end subscript  
  3. f open parentheses x close parentheses greater than g left parenthesis x right parenthesis for all x greater than x subscript 0 end subscript  
  4. None of these  

The correct answer is: f open parentheses x close parentheses greater than g left parenthesis x right parenthesis for all x greater than x subscript 0 end subscript


    Consider the function ϕ open parentheses x close parentheses equals f open parentheses x close parentheses minus g left parenthesis x right parenthesis on the interval open square brackets x subscript 0 end subscript comma x close square brackets. Clearly, ϕ left parenthesis x right parenthesis satisfies conditions of Lagrange’s theorem on open square brackets x subscript 0 end subscript comma x close square brackets. Therefore, there exists c element of left parenthesis x subscript 0 end subscript comma x right parenthesis such that
    ϕ open parentheses x close parentheses minus ϕ open parentheses x subscript 0 end subscript close parentheses equals ϕ to the power of ´ end exponent open parentheses c close parentheses open parentheses x minus x subscript 0 end subscript close parentheses

    rightwards double arrow ϕ open parentheses x close parentheses equals ϕ ´ left parenthesis c right parenthesis left parenthesis x minus x subscript 0 end subscript right parenthesis…(i)

    Now,

    ϕ open parentheses x close parentheses equals f open parentheses x close parentheses minus g left parenthesis x right parenthesis

    rightwards double arrow ϕ to the power of ´ end exponent open parentheses x close parentheses equals f to the power of ´ end exponent open parentheses x close parentheses minus g ´ left parenthesis x right parenthesis

    rightwards double arrow ϕ to the power of ´ end exponent open parentheses c close parentheses equals f to the power of ´ end exponent open parentheses c close parentheses minus g to the power of ´ end exponent open parentheses c close parentheses greater than 0 open square brackets because f to the power of ´ end exponent open parentheses x close parentheses greater than g to the power of ´ end exponent open parentheses x close parentheses f o r blank x greater than x subscript 0 end subscript close square brackets

    From (i), we have

    ϕ open parentheses x close parentheses greater than 0for all x greater than x subscript 0 end subscript open square brackets because ϕ to the power of ´ end exponent open parentheses c close parentheses greater than 0 blank a n d blank x minus x subscript 0 end subscript greater than 0 close square brackets

    rightwards double arrow f open parentheses x close parentheses minus g open parentheses x close parentheses greater than 0for all x greater than x subscript 0 end subscript

    rightwards double arrow f open parentheses x close parentheses greater than g left parenthesis x right parenthesisfor all x greater than x subscript 0 end subscript

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