The coefficient of \(x^3\) in the expansion of \((1-ax)^{\frac25}\) is \(1\).

(a) Find the value of \(a\).

(b) Hence, find the coefficient of \(x^4\) in the expansion of \((2x+1)(1-ax)^{\frac25}\).

(c) State the set of values of \(x\) for which the expansion in 4(b) is valid.