(a+b+c)^3=27
(a+b+c)^3=(a+b)^3+3(a+b)^2c+3(a+b)c^2+c^3=a^3+b^3+c^3+3a^2b+3a^2c+3b^2a+3b^2c+3c^2a+3c^2b+6abc= =a^3+b^3+c^3-3abc+3(a^2b+a^2c+b^2a+b^2c+c^2a+c^2b+3abc)= =a^3+b^3+c^3-3abc+3(a(ab+ac+bc)+b(ab+ac+bc)+c(ab+ac+bc))=
=a^3+b^3+c^3-3abc+3(a+b+c)(ab+ac+bc)= a^3+b^3+c^3-3abc+18
a^3+b^3+c^3-3abc+18=27
a^3+b^3+c^3-3abc=9
(a+b+c)^3=27
(a+b+c)^3=(a+b)^3+3(a+b)^2c+3(a+b)c^2+c^3=a^3+b^3+c^3+3a^2b+3a^2c+3b^2a+3b^2c+3c^2a+3c^2b+6abc= =a^3+b^3+c^3-3abc+3(a^2b+a^2c+b^2a+b^2c+c^2a+c^2b+3abc)= =a^3+b^3+c^3-3abc+3(a(ab+ac+bc)+b(ab+ac+bc)+c(ab+ac+bc))=
=a^3+b^3+c^3-3abc+3(a+b+c)(ab+ac+bc)= a^3+b^3+c^3-3abc+18
a^3+b^3+c^3-3abc+18=27
a^3+b^3+c^3-3abc=9