If $a^2+b^2-a b-a-b+1 \leq 0, a, b \leq R$, then $a+b$ is equal to:

5 2 9 4

2

Given that $a^2+b^2-a b-a-b+1 \leq 0$ $\Rightarrow 2 a^2+2 b^2-2 a b-2 a-2 b+2 \leq 0$ $(a-b)^2+(a-1)^2+(b-1)^2 \leq 0$ ⇒ a = 1 and b = 1 Hence (2) is the correct answer.