x*x-2x y*y-4y 5=0
25+40х+16х^2=36х+45-84х^2-105х
(2а+4b) (2a-4b) = 4a²-16b²
81²-2,56 = (9+1,6)(9-1,6)
0,25x²-0,49y² = (0,5x+0,7y)(0,5x-0,7y)