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Math GRE subject test question

  1. May 23, 2015 #1
    Let k be the number of real solutions to the equation e^x + x - 2 = 0 in the interval [0,1] and let n be the number of real solutions NOT on the interval [0,1]. Which of the following are true?

    A) k = 0 and n = 1
    B) k = 1 and n = 0
    C) k = n = 1
    D) k > 1
    E) n > 1

    Can anyone help me understand this? I'm thinking that the answer is B, because if x = 1 then the equation will be greater than zero, and the equation starts below the x-axis. So somewhere in the interval [0,1], the line must cross the x=axis. Hence B is the answer. Is this correct?
     
  2. jcsd
  3. May 23, 2015 #2

    Mark44

    Staff: Mentor

    Maybe or maybe not. The real question is whether the graph of the function f(x) = ex + x - 2 crosses the x-axis outside the interval [0, 1]. If it does so once, then C would be the answer.

    How do you know for certain that there is only one x-intercept? Hint: take the derivative of f.

    Minor quibble: an equation is not greater than zero, less than zero, or equal to zero. An equation is a statement that two quantities or expressions are equal. An inequality is a statement that one expression is larger than, or smaller than, another.
     
  4. May 25, 2015 #3

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    I think it is simpler to convert [itex]e^x+ x- 2= 0[/itex] to [itex]e^x= 2- x[/itex].

    Now, even a rough graph of [itex]y= e^x[/itex] and [itex]y= 2- x[/itex] will give the answer.
     
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