a= 81xcarré+45x - 45x - 25 = 81 x² - 25 = (9x)² - (5)²
b = (5x-9)² = 25x² +90x +81= (5x)² + 9(10x + 9)
c= (10x+3)² = 100x² + 60x +9 = (10x)² + 3(20x + 3)
d =(3x+8) (6x - 8) = 18x² -24x + 48 x -64 = 18x² +24x -64 = 2(9x² +12x -32)
e = (5x -6)(5x +6)= 25x² +30x - 30 x - 36 = 25x² -36
f = (10x -5)² - (7x+7)² = 100x² -100x +25 - 49x² +98x + 49 = 51x² -2x + 74