They just factored it.
ok just a question about the working out for this problem,
4x^2 - 8xy - 9y^2 = -637
= 8x - 8y - 8x d/dx(y) - d/dy(9y^2)dy/dx
= 8x - 8y - 8x dy/dx - 18y dy/dx
but where does the + sign come from??? can anyone explicitly show where this comes from.. I think it must come from the product rule but I just cant see it...
A local minimum is a point c such that for all x in an open interval containing c.
A local maximum is a point c such that for all x in an open interval containing c.
Best points to focus on is the critical points, the points where the derivative of the function does not exist or equals 0. Plug these critical points in the original function.