I am getting values for x and y

x= sqrt(7/8) and y= 7/(sqrt(7/8))

the equations i get:

2xy+1=ylambda

x^2+1=xlambda

the equation i get from setting lambdas equal to eacher : (2xy+1)/y = (x^2+1)/x

solve for y in terms of x, y = x/(x^2+1-2x^2)

plugging y (x) into my constraint of xy=7 , i solve for x=+/- sqrt(7/8). and i also get a value for y. Plugging these x,y for f(x,y), i get a value... why is the answer in the answer key DNE??