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(15-x)^(0.75)=13.9 equation

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Numerical solution:

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The solution

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        3/4   139
(15 - x)    = ---
               10
$$\left(15 - x\right)^{\frac{3}{4}} = \frac{139}{10}$$
Detail solution
Given the equation
$$\left(15 - x\right)^{\frac{3}{4}} = \frac{139}{10}$$
Because equation degree is equal to = 3/4 - does not contain even numbers in the numerator, then
the equation has single real root.
We raise the equation sides to 4/3-th degree:
We get:
$$\left(\left(15 - x\right)^{\frac{3}{4}}\right)^{\frac{4}{3}} = \left(\frac{139}{10}\right)^{\frac{4}{3}}$$
or
$$15 - x = \frac{139 \cdot 10^{\frac{2}{3}} \sqrt[3]{139}}{100}$$
Expand brackets in the right part
15 - x = 139*10^2/3*139^1/3/100

Move free summands (without x)
from left part to right part, we given:
$$- x = -15 + \frac{139 \cdot 10^{\frac{2}{3}} \sqrt[3]{139}}{100}$$
Divide both parts of the equation by -1
x = -15 + 139*10^(2/3)*139^(1/3)/100 / (-1)

We get the answer: x = 15 - 139*10^(2/3)*139^(1/3)/100

The final answer:
$$x_{1} = - \frac{139 \cdot 10^{\frac{2}{3}} \sqrt[3]{139}}{100} + 15$$
The graph
Sum and product of roots [src]
sum
           2/3 3 _____
     139*10   *\/ 139 
15 - -----------------
            100       
$$- \frac{139 \cdot 10^{\frac{2}{3}} \sqrt[3]{139}}{100} + 15$$
=
           2/3 3 _____
     139*10   *\/ 139 
15 - -----------------
            100       
$$- \frac{139 \cdot 10^{\frac{2}{3}} \sqrt[3]{139}}{100} + 15$$
product
           2/3 3 _____
     139*10   *\/ 139 
15 - -----------------
            100       
$$- \frac{139 \cdot 10^{\frac{2}{3}} \sqrt[3]{139}}{100} + 15$$
=
           2/3 3 _____
     139*10   *\/ 139 
15 - -----------------
            100       
$$- \frac{139 \cdot 10^{\frac{2}{3}} \sqrt[3]{139}}{100} + 15$$
15 - 139*10^(2/3)*139^(1/3)/100
Rapid solution [src]
                2/3 3 _____
          139*10   *\/ 139 
x1 = 15 - -----------------
                 100       
$$x_{1} = - \frac{139 \cdot 10^{\frac{2}{3}} \sqrt[3]{139}}{100} + 15$$
x1 = -139*10^(2/3)*139^(1/3)/100 + 15
Numerical answer [src]
x1 = -18.4210225678832
x1 = -18.4210225678832