\[\boxed{\mathbf{9.}}\]
\[\textbf{а)}\ 26 - 1 = 25\]
\[42 - 1 = 41\]
\[68 - 1 = 67\]
\[84 - 1 = 83\]
\[\textbf{б)}\ 50 - 1 = 49\]
\[100 - 1 = 99\]
\[71 - 1 - 1 = 70 - 1 = 69\]
\[90 - 1 - 1 = 89 - 1 = 88\]