SOLUTION: Find x, (7 + √x)^⅓ + (7 - √x)^⅓ = 2

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Question 1205857: Find x,
(7 + √x)^⅓ + (7 - √x)^⅓ = 2

Answer by ikleyn(52777) About Me  (Show Source):
You can put this solution on YOUR website!
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Find x,
(7 + √x)^⅓ + (7 - √x)^⅓ = 2
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Your starting equation is

    %287%2Bsqrt%28x%29%29%5E%281%2F3%29 + %287-sqrt%28x%29%29%5E%281%2F3%29 = 2.    (1)


Let  a = %287%2Bsqrt%28x%29%29%5E%281%2F3%29;  b = %287-sqrt%28x%29%29%5E%281%2F3%29.


Raise both sides of equation (1) to degree 3.

Use

    %28a+%2B+b%29%5E3 = a%5E3+%2B+b%5E3 + 3ab*(a+b),


    a%5E3+%2B+b%5E3 = %287%2Bsqrt%28x%29%29  + %287-sqrt%28x%29%29 = 14,

    a + b = 2   <<<---===  given by equation,

    ab = %2849+-+x%29%5E%281%2F3%29.


You will get

    14+%2B+3%2A%2849-x%29%5E%281%2F3%29%2A2 = 8.


Simplify and find x

    %2849+-+x%29%5E%281%2F3%29 = %288-14%29%2F6,

    %2849+-+x%29%5E%281%2F3%29 = -1.


Raise both sides of the last equation to degree 3

    49 - x = -1,

    49 + 1 = x,

     x     = 50.


ANSWER.  x = 50.


CHECK.   %287+%2B+sqrt%2850%29%29%5E%281%2F3%29 + %287+-+sqrt%2850%29%29%5E%281%2F3%29 = 2.414213562 + (-0.414213562) = 2.0000000

Solved.