Rearrange lens formula - cannot find explicit solution

Hey guys, trying to arrange the symmetrical lens formula for overlapping circles for the offset variable (eqn found here - http://mathworld.wolfram.com/Lens.html)

Can't get an answer and can't see what im doing wrong

Any advice would be appreciated. The variable to be the subject in this case is d

>> syms a
>> syms d
>> syms A
>> solve(a^2*pi-2*a^2*atan(d/sqrt(4*a^2-d^2))-0.5*d*sqrt(4*a^2-d^2)==A, d)
Warning: Cannot find explicit solution. 
> In solve (line 316)