Question

x^2-1-2y-y^2 factorise

Answer 1

${x}^{2} - ( {y}^{2} + 2y + 1) \\ {x}^{2} - ( {y + 1)}^{2} \\ because \: ( {x + y)}^{2} = {x}^{2} + 2xy + {y}^{2} \\ {x}^{2} - {y}^{2} = (x - y)(x + y) \\ apply \: the \: identity \\ (x - y - 1)(x + y + 1)$