The function is
Apply derivative on each side with respect to .
\
.
The condition for horizontal tangent is
Substitute in the equation
.
and
Therefore the horizontal tangent the point are and
.
The condition for vertical tangent is .
Substitute in the equation
.
and
.
Therefore the vertical tangent the point are and
.
Hence the horizontal tangent the point are and
.
And the vertical tangent the point are and
.