\[Страница\ 3.\]
\[\boxed{\mathbf{1.}}\]
\[Первое\ слагаемое\ \]
\[увеличивается\ на\ 1,\ \]
\[второе\ слагаемое\ \]
\[уменьшается\ на\ 1.\]
\[42 + 9 = 51\]
\[43 + 8 = 51\]
\[44 + 7 = 51\]
\[45 + 6 = 51\]
\[34 + 6 = 40\]
\[35 + 5 = 40\]
\[34 + 6 = 40\]
\[33 + 7 = 40\]
\[91 + 8 = 99\]
\[92 + 7 = 99\]
\[93 + 6 = 99\]
\[94 + 5 = 99\]
\[\boxed{\mathbf{2.}}\]
\[\boxed{\mathbf{3.}}\]