f : ℝ → ℝ, f(x) = -9x-6
S = {(a, b) ∈ ℝ×ℝ| f(a) = b ∧ a+b = -11}
① f(a) = b ⇔ -9a-6 = b ⇔ 9a+b = -6
② a+b = -11 ⇔ b = -11-a
⇒ 9a-11-a = -6 ⇔ 8a = 5 ⇔ a = 5/8
⇒ b = -11 - 5/8 = (-88 - 5)/8 = -93/8
[tex]\Rightarrow S = \left(\dfrac{5}{8}, -11\dfrac{5}{8}\right)[/tex]
