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Express y in terms of y where y^2+x^2=2x+4y

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Numerical solution:

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The solution

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 2    2            
y  + x  = 2*x + 4*y
$$x^{2} + y^{2} = 2 x + 4 y$$
Detail solution
Move right part of the equation to
left part with negative sign.

The equation is transformed from
$$x^{2} + y^{2} = 2 x + 4 y$$
to
$$\left(- 2 x - 4 y\right) + \left(x^{2} + y^{2}\right) = 0$$
This equation is of the form
a*y^2 + b*y + c = 0

A quadratic equation can be solved
using the discriminant.
The roots of the quadratic equation:
$$y_{1} = \frac{\sqrt{D} - b}{2 a}$$
$$y_{2} = \frac{- \sqrt{D} - b}{2 a}$$
where D = b^2 - 4*a*c - it is the discriminant.
Because
$$a = 1$$
$$b = -4$$
$$c = x^{2} - 2 x$$
, then
D = b^2 - 4 * a * c = 

(-4)^2 - 4 * (1) * (x^2 - 2*x) = 16 - 4*x^2 + 8*x

The equation has two roots.
y1 = (-b + sqrt(D)) / (2*a)

y2 = (-b - sqrt(D)) / (2*a)

or
$$y_{1} = \frac{\sqrt{- 4 x^{2} + 8 x + 16}}{2} + 2$$
$$y_{2} = 2 - \frac{\sqrt{- 4 x^{2} + 8 x + 16}}{2}$$
Rapid solution [src]
             _______________________________________________________________                                                                            _______________________________________________________________                                                                   
            /                                                             2     /     /                               2        2             \\        /                                                             2     /     /                               2        2             \\
         4 /                           2   /      2        2             \      |atan2\2*im(x) - 2*im(x)*re(x), 4 + im (x) - re (x) + 2*re(x)/|     4 /                           2   /      2        2             \      |atan2\2*im(x) - 2*im(x)*re(x), 4 + im (x) - re (x) + 2*re(x)/|
y1 = 2 - \/   (2*im(x) - 2*im(x)*re(x))  + \4 + im (x) - re (x) + 2*re(x)/  *cos|-------------------------------------------------------------| - I*\/   (2*im(x) - 2*im(x)*re(x))  + \4 + im (x) - re (x) + 2*re(x)/  *sin|-------------------------------------------------------------|
                                                                                \                              2                              /                                                                            \                              2                              /
$$y_{1} = - i \sqrt[4]{\left(- 2 \operatorname{re}{\left(x\right)} \operatorname{im}{\left(x\right)} + 2 \operatorname{im}{\left(x\right)}\right)^{2} + \left(- \left(\operatorname{re}{\left(x\right)}\right)^{2} + 2 \operatorname{re}{\left(x\right)} + \left(\operatorname{im}{\left(x\right)}\right)^{2} + 4\right)^{2}} \sin{\left(\frac{\operatorname{atan_{2}}{\left(- 2 \operatorname{re}{\left(x\right)} \operatorname{im}{\left(x\right)} + 2 \operatorname{im}{\left(x\right)},- \left(\operatorname{re}{\left(x\right)}\right)^{2} + 2 \operatorname{re}{\left(x\right)} + \left(\operatorname{im}{\left(x\right)}\right)^{2} + 4 \right)}}{2} \right)} - \sqrt[4]{\left(- 2 \operatorname{re}{\left(x\right)} \operatorname{im}{\left(x\right)} + 2 \operatorname{im}{\left(x\right)}\right)^{2} + \left(- \left(\operatorname{re}{\left(x\right)}\right)^{2} + 2 \operatorname{re}{\left(x\right)} + \left(\operatorname{im}{\left(x\right)}\right)^{2} + 4\right)^{2}} \cos{\left(\frac{\operatorname{atan_{2}}{\left(- 2 \operatorname{re}{\left(x\right)} \operatorname{im}{\left(x\right)} + 2 \operatorname{im}{\left(x\right)},- \left(\operatorname{re}{\left(x\right)}\right)^{2} + 2 \operatorname{re}{\left(x\right)} + \left(\operatorname{im}{\left(x\right)}\right)^{2} + 4 \right)}}{2} \right)} + 2$$
             _______________________________________________________________                                                                            _______________________________________________________________                                                                   
            /                                                             2     /     /                               2        2             \\        /                                                             2     /     /                               2        2             \\
         4 /                           2   /      2        2             \      |atan2\2*im(x) - 2*im(x)*re(x), 4 + im (x) - re (x) + 2*re(x)/|     4 /                           2   /      2        2             \      |atan2\2*im(x) - 2*im(x)*re(x), 4 + im (x) - re (x) + 2*re(x)/|
y2 = 2 + \/   (2*im(x) - 2*im(x)*re(x))  + \4 + im (x) - re (x) + 2*re(x)/  *cos|-------------------------------------------------------------| + I*\/   (2*im(x) - 2*im(x)*re(x))  + \4 + im (x) - re (x) + 2*re(x)/  *sin|-------------------------------------------------------------|
                                                                                \                              2                              /                                                                            \                              2                              /
$$y_{2} = i \sqrt[4]{\left(- 2 \operatorname{re}{\left(x\right)} \operatorname{im}{\left(x\right)} + 2 \operatorname{im}{\left(x\right)}\right)^{2} + \left(- \left(\operatorname{re}{\left(x\right)}\right)^{2} + 2 \operatorname{re}{\left(x\right)} + \left(\operatorname{im}{\left(x\right)}\right)^{2} + 4\right)^{2}} \sin{\left(\frac{\operatorname{atan_{2}}{\left(- 2 \operatorname{re}{\left(x\right)} \operatorname{im}{\left(x\right)} + 2 \operatorname{im}{\left(x\right)},- \left(\operatorname{re}{\left(x\right)}\right)^{2} + 2 \operatorname{re}{\left(x\right)} + \left(\operatorname{im}{\left(x\right)}\right)^{2} + 4 \right)}}{2} \right)} + \sqrt[4]{\left(- 2 \operatorname{re}{\left(x\right)} \operatorname{im}{\left(x\right)} + 2 \operatorname{im}{\left(x\right)}\right)^{2} + \left(- \left(\operatorname{re}{\left(x\right)}\right)^{2} + 2 \operatorname{re}{\left(x\right)} + \left(\operatorname{im}{\left(x\right)}\right)^{2} + 4\right)^{2}} \cos{\left(\frac{\operatorname{atan_{2}}{\left(- 2 \operatorname{re}{\left(x\right)} \operatorname{im}{\left(x\right)} + 2 \operatorname{im}{\left(x\right)},- \left(\operatorname{re}{\left(x\right)}\right)^{2} + 2 \operatorname{re}{\left(x\right)} + \left(\operatorname{im}{\left(x\right)}\right)^{2} + 4 \right)}}{2} \right)} + 2$$
y2 = i*((-2*re(x)*im(x) + 2*im(x))^2 + (-re(x)^2 + 2*re(x) + im(x)^2 + 4)^2)^(1/4)*sin(atan2(-2*re(x)*im(x) + 2*im(x, -re(x)^2 + 2*re(x) + im(x)^2 + 4)/2) + ((-2*re(x)*im(x) + 2*im(x))^2 + (-re(x)^2 + 2*re(x) + im(x)^2 + 4)^2)^(1/4)*cos(atan2(-2*re(x)*im(x) + 2*im(x), -re(x)^2 + 2*re(x) + im(x)^2 + 4)/2) + 2)