Question

Given a + b + c = 5 and a² + b² + c² = 9, find the value of ab + ac + bc

Answer 1

ab+ac+bc=8

Answer 2

procedure:
\[(a+b+c)^2=a^2+b^2+c^2+2(ab+ac+bc)\]
\[5^2=9+2(ab+ac+bc)\]
\[ab+ac+bc=8\]