Кто-нибудь, помогите решить номер 6.

a^2 + b^2 + c^2 = 19 --------> a^2 + b^2 = 19 - c^2 ------> a^2 + 2ab + b^2 = 19 - c^2 + 2ab
a + b - c = 7 --------------------> a + b = 7 + c
=>
(a + b)^2 = 19 - c^2 + 2ab
(a + b)^2 = 49 + 14c + c^2
=>
19 - c^2 + 2ab = 49 + 14c + c^2
ab = (30 + 14c + 2c^2)\2 = 15 + 7c + c^2

b^2 + c^2 = 19 - a^2 --------> (b - c)^2 = 19 - a^2 - 2bc
b = c = 7 - a -------------------> (b - c)^2 = 49 - 14a + a^2
19 - a^2 - 2bc = 49 - 14a + a^2
bc = (-30 - 2a^2 + 14a)\2 = -15 - a^2 + 7a

a^2 + c^2 = 19 - b^2 -------> (a - c)^2 = 19 - b^2 - 2ac
(a - c) = 7 - b ----------------> (a - c)^2 = 49 - 14b + b^2
=>
19 - b^2 - 2ac = 49 - 14b + b^2
ac = (-30 - 2b^2 + 14b)\2 = -15 - b^2 + 7b
=>

ab - bc - ac =
= (15 + 7c + c^2) - (-15 - a^2 + 7a) - (-15 - b^2 + 7b) =
= 45 + 7*(c - a - b) + a^2 + b^2 + c^2