4 The equation of a curve is
$$\sqrt { } x + \sqrt { } y = \sqrt { } a$$
where \(a\) is a positive constant.
- Express \(\frac { \mathrm { d } y } { \mathrm {~d} x }\) in terms of \(x\) and \(y\).
- The straight line with equation \(y = x\) intersects the curve at the point \(P\). Find the equation of the tangent to the curve at \(P\).