[Problem of the day][数学挑战][Powers and Indices][指数和幂运算][1] Mathematics Algebra

casperyc 2020-4-28 1045

Find all real numbers $x$ for which:

$$\huge (x^2-7x+11)^{(x^2-13x+42)}=1$$


What about

$$\huge (x^2-7x+11)^{(x^2+13x-42)}=1$$

最新回复 (2)
  • casperyc的马甲 2020-4-28
    引用 2

    For the equation

    $$\huge (x^2-7x+11)^{(x^2-13x+42)}=1$$

    Let's consider the following cases.

    Case 1:

    $$ \large 1^y = 1 \quad \implies \quad x=2,5$$

    Case 2:

    $$ \large \underbrace{y^0}_{y \neq 0} = 1 \quad \implies \quad x=6,7$$

    Case 3:

    $$ \large \underbrace{(-1)^{2n}}_{n \in \mathbb{Z}} = 1 \quad \implies \quad x=3,4$$

    For the last case, we need to check the that the (exponent) power is even when $x=3$,

    $$3^2-13 \times (3)+42=12$$

    and when $x=4$,

    $$4^2-13 \times (4)+42=6$$

    which works!

    Final answer:

    $$ x=2,3,4,5,6,7$$

     

    Checking with Mathematica:

    Solve[(x^2 - 7*x + 11)^(x^2 - 13 x + 42) == 1, x, Reals]
    
    >> {{x -> 2}, {x -> 3}, {x -> 4}, {x -> 5}, {x -> 6}, {x -> 7}}
  • casperyc的马甲 2020-4-29
    引用 3

    For

    $$\huge (x^2-7x+11)^{(x^2+13x-42)}=1$$

    Similarly, we get

    $$x=2,3,4,5$$ and

    $$x=\frac{ -13 \pm \sqrt{337} }{2} $$

    Using computer to check it, we get

    Solve[(x^2 - 7*x + 11)^(x^2 + 13 x - 42) == 1, x, Reals]
    
    >> {{x -> 2}, {x -> 3}, {x -> 4}, {x -> 5}, {x -> 1/2 (-13 - Sqrt[337])}, {x -> 1/2 (-13 + Sqrt[337])}}
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