Question: Circle of maximum radius tangent to both curves

Find the circle of maximum radius that is tangent to both curves. 
There are no other conditions or restrictions.
Equations of curves

 (x1-1/2)^4+x1*x2+2*x2^2-1=0; 
 (x1-sin(x1))^2+(x2-sin(x2))^2-1=0;
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