(a^2b^2+ab)^2-(ab+1)^2 =[(a^2b^2+ab)+(ab+1)][(a^2b^2+ab)-(ab+1)] =(a^2b^2+2ab+1)(a^2b^2-1) =(ab+1)^2(ab+1)(ab-1) =(ab+1)^3(ab-1)