4 It is given that \(y = 5 ^ { x - 1 }\).
- Show that \(x = 1 + \frac { \ln y } { \ln 5 }\).
- Find an expression for \(\frac { \mathrm { d } x } { \mathrm {~d} y }\) in terms of \(y\).
- Hence find the exact value of the gradient of the curve \(y = 5 ^ { x - 1 }\) at the point (3, 25).