I do not know yet the answer
But! I solved such examples, but the Number A always was greater than B (a> b)
Example:
√ (11-4√7) = √ ((2-√7) ^ 2) =! 2-√7! = √7 -2
how to find A and B? Selection method. We take -4√7 divide by 2 (according to the formula (a-b) ^ 2 = a ^ 2 -2ab + b ^ 2), we get -2√7, where 2 = A, √7 = B (we check by substitution in formula, given to us the numbers are suitable).
Only here such a trick with my example does not pass ... ((