А) x^2+4xy+5y^2+4y+2 = (x^2 + 4xy + 4y^2) + (y^2 +4y + 4) - 2 =
(x + 2y)^2 + (y + 2)^2 - 2
(x + 2y)^2 = min = 0 при x + 2y = 0 ⇔ x = -2y
(y + 2) ^2 = min = 0 при y = - 2
x = -2y = -2 * (-2) = 4
0 + 0 - 2 = -2
б) (x^2+4xy+4y^2)+2x+4y+2 = (x + 2y)^2 + 2(x + 2y) + 1 + 1 =
(x + 2y + 1)^2 + 1
минимально при x + 2y + 1 = 0
x = -2y - 1
y ∈ R
0^2 + 1 = 1
в) x^2+y^2+z^2+2xy+2x+2y-4z+12 =
(x^2 + 2xy + y^2) + 2(x + y) + 1 + (z^2 - 4z + 4) + 7 =
(x + y + 1)^2 + (z - 2)^2 + 7
аналогично
z = 2
x ∈ R
y = -1 - x
значение = 7