$\"\"$

(a) The function is $\"\"$ .

Differentiate on each side with respect to $\"\"$ .

$\"\"$

$\"\"$ .

Slope of the tangent is derivative of the function.

$\"\"$ .

The curve has horizontal tangent when slope of the tangent is zero.

Therefore, $\"\"$ .

$\"\"$

$\"\"$ .

$\"\"$

Observe the graph, we have slope of the tangent is zero at $\"\"$ .

Hence the curve has horizontal tangent at $\"\"$ .

The above graph is consistent with the result of part (b).

$\"\"$

(a) $\"\"$ .

(b) $\"\"$ .