Question

Use implicit differentiation to find the points where the circle defined by x^2+y^2−6x−4y=-4 has horizontal and vertical tangent lines. List your answers as points in the form (a,b).

Answer 1

Implicit differentiation: 2x+2ydy/dx-6-4dy/dx=0. When dy/dx=0, we have a horizontal tangent.
So 2x-6=0 and x=3. To find y we solve 9+y²-18-4y=-4, y²-4y-5=0=(y-5)(y+1), so the points are (3,5) and (3,-1).
2xdx/dy+2y-6dx/dy-4=0. When dx/dy=0 we have a vertical tangent, so 2y=4, y=2, and x²+4-6x-8=-4, x²-6x=0, x=0 and 6.
The points are (0,2) and (6,2).