1 - x² + 10xy - 25y² = 1 - (x² - 10xy + 25y²) = 1 - (x - 5y)² =
= (1 - x + 5y)(1 + x - 5y)
1-x^2+10xy-25y^2 = 1 - ( x-5 )^2 = (1 - x - 5y) * (1 + x - 5y)
