028.38. a) $(a - c)(a + c) - (a - 2c)^2$; б) $(x - 4)(x + 4) - (x + 8)(x - 8)$; в) $(3b - 1)(3b + 1) - (b - 5)(b + 5)$; г) $(m + 3n)^2 + (m + 3n)(m - 3n)$.

a) $(a-c)(a+c) - (a-2c)^2 = (a^2 - c^2) - (a^2 - 4ac + 4c^2) = a^2 - c^2 - a^2 + 4ac - 4c^2 = 4ac - 5c^2$ б) $(x-4)(x+4) - (x+8)(x-8) = (x^2 - 16) - (x^2 - 64) = x^2 - 16 - x^2 + 64 = 48$ в) $(3b-1)(3b+1) - (b-5)(b+5) = (9b^2 - 1) - (b^2 - 25) = 9b^2 - 1 - b^2 + 25 = 8b^2 + 24$ г) $(m+3n)^2 + (m+3n)(m-3n) = (m^2 + 6mn + 9n^2) + (m^2 - 9n^2) = m^2 + 6mn + 9n^2 + m^2 - 9n^2 = 2m^2 + 6mn$