k=1/[x+sqrt(x^2+y^2)]
color(white)("XXX")rarr x+sqrt(x^2+y^2)=1/k
color(white)("XXX")rarr sqrt(x^2+y^2)=1/k-x
color(white)("XXX")rarr x^2+y^2=1/k^2-2x/k+x^2
color(white)("XXX")rarr y^2=1/k^2-(2kx)/k^2
color(white)("XXX")rarr y=+-sqrt(1-2kx)/k
Assuming k is a (non-zero) constant
and y is dependent on the variable x
color(white)("XXX")y_x=+-sqrt(1-2kx)/k