[填空题]
The function f is d+sn k.i4 o/jht2hck7m -8jnv htiyvwv6)/t0 e efined by $f(x)=e^{2 x}-4 e^{x}+2$ , for $ x \in \mathbb{R}$, $x \leq a$ , where a \in \mathbb{R} . Part of the graph of y=f(x) is shown in the following diagram.
1. Find the largest value of a such that f has an inverse function.
2. For this value of a , find an expression for $f^{-1}(x)$ stating its domain
