L(x, y, λ )= x – y + λ (x² – 2 y² − 10) ; ∂L / ∂x = 1 + 2 λ x = 0 ; λ = -1 / (2 x) ; ∂L / ∂y = -1 – 4 λ y = 0 ; λ = -1 / (4 y) ; 2 x = 4 y ; x = 2 y ; ∂L / ∂λ = 0 ; x² – 2 y² = 10 ; (2 y)² – 2 y² = 10 ; 2 y² = 10 ; y = {-5^1/2 , 5^1/2}; x = {-2 5^1/2 , 2 5^1/2} ; f(x, y) = x – y = 5^1/2 > 0. pg. 193. Correct answer is D.