Question

разложить на множители:
4-x^2+2xy-y^2

Answer 1

$4-x^2+2xy-y^2=4-(x^2-2xy+y^2)=\\\\=4-(x-y)^2=[\, A^2-B^2=(A-B)(A+B)\,]=\\\\=(4-x+y)(4+x-y)$