The function is .

The function has horizontal tangent line which has a zero slope.

Slope of the tangent line is the derivative of the function.

Equate derivative of the function to zero.

.

Apply derivative on each side with respect to .

Quotient rule of derivatives : .

.

.

Equate derivative function to zero.

Substitute in the function.

Point of tangency is .

Hence the point at which the function has horizontal tangent line is .

The point at which the function has horizontal tangent line is .