Problem: simplify : 81 x 81 + 68 x 68 - 2 x 81 x 68 .
Rationale: = ( 81 ) ^ 2 + ( 68 ) ^ 2 – 2 x 81 x 68 = a ^ 2 + b ^ 2 – 2 ab , where a = 81 , b = 68 = ( a - b ) ^ 2 = ( 81 – 68 ) ^ 2 = ( 13 ) ^ 2 = 169 . answer is a .
annotated_formula: add(81, 81)