Question

If -xy – 5+ y^2+ x^2= 0 and it is known that dy/dx= y-2x/-x+2y, find all

coordinate points on the curve where x = -1 and the line tangent to the
curve is horizontal, or state that no such points exist.

Answer 1

There is no point of the form (-1, y) on the curve where the tangent is horizontal

Explanation:

Notice that when x = - 1. then dy/dx becomes:

dy/dx= (y+2) / (2y+1)

therefore, to request that the tangent is horizontal we ask for the y values that make dy/dx equal to ZERO:

0 = ( y + 2) / (2 y + 1)

And we obtain y = -2 as the answer.

But if we try the point (-1, -2) in the original equation, we find that it DOESN'T belong to the curve because it doesn't satisfy the equation as shown below:

(-1)^2 + (-2)^2 - (-1)*(-2) - 5 = 1 + 4 + 2 - 5 = 2 (instead of zero)

Then, we conclude that there is no horizontal tangent to the curve for x = -1.