$a)$
$(x-2).5-5=5$
⇒$(x-2).5=10$
⇒$x-2=2$
⇒$x=4$
Vậy $x=4$
$b)$
$x^{200}$ $=x$
⇒$x^{200}$$-x$$=0$
⇒$x^{199}$ $(x-1)=0$
⇒\(\left[ \begin{array}{l}x^{199}=0\\x-1=0\end{array} \right.\)
⇒\(\left[ \begin{array}{l}x=0\\x=-1\end{array} \right.\)
Vậy $x∈0,-1$