7) m^2 - n^2 + m + n = (m + n)(m - n) + (m + n) = (m + n)(m - n + 1)
8) x^2 - y^2 + 14y - 49 = x^2 - (y^2 - 14y + 49) = x^2 - (y - 7)^2 =
= (x - y + 7)(x + y - 7)
9) 81a^4 - 1 = (9a^2 +1)(9a^2 - 1) = (9a^2 +1)(3a + 1)(3a - 1)
10) 49x - x^2 = 0
x(49 - x) = 0 ----> x1=0 x2=49
11) x^3 + 3x^2 - x - 3 = 0
x^2(x + 3) - (x + 3) = 0
(x + 3)(x^2 - 1) = 0
(x + 3)(x + 1)(x - 1) = 0 ----> x1 = -3; x2= -1; x3 = 1
12) (x^2 - 2)^2 - 4(x^2 - 2) + 4 = (x^2 - 2 - 2)^2 = (x^2 -4)^2 = (x - 2)^2 * (x + 2)^2