1) (xy-0.2)(xy+0.2); 2) (0.4-x)(0.4+x) => x=±0.4; 3)(12-7n)(12+7n); 4)(ab-c)(ab+c); 5) (pq-2k)(pq+2k); 6) 30²-1²=900-1=899; 7) (55-45)(55+45)=10*100=1000; 8)(41-31)(41+31)=10*72=720; 9) (76-24)(76+24)=52*100=5200; 10)(37-23)(37+23)=14*60=840; 11)x²=0.04; x=±0.2; 12)x²=81; x=±9; 13) 4a²b²-c²; 14)16a²-25b²; 15)16-9x²y²; 16)4x²y²-1; 17)a⁴-25; 18) 81x²-y⁴