Đáp án:
$\begin{array}{l}
A\left( {a;x} \right);B\left( {b;y} \right)\\
AB = \sqrt {{{\left( {a - b} \right)}^2} + {{\left( {x - y} \right)}^2}} \\
a)A\left( {1;1} \right);B\left( {5;4} \right)\\
\Leftrightarrow AB = \sqrt {{{\left( {1 - 5} \right)}^2} + {{\left( {1 - 4} \right)}^2}} = \sqrt {{4^2} + {3^2}} = \sqrt {25} = 5\\
b)\\
M\left( { - 2;2} \right);N\left( {3;5} \right)\\
\Leftrightarrow MN = \sqrt {{{\left( { - 2 - 3} \right)}^2} + {{\left( {2 - 5} \right)}^2}} = \sqrt {{5^2} + {3^2}} = \sqrt {34} \\
c)\\
P\left( {{x_1};{y_1}} \right);Q\left( {{x_2};{y_2}} \right)\\
\Leftrightarrow PQ = \sqrt {{{\left( {{x_1} - {x_2}} \right)}^2} + {{\left( {{y_1} - {y_2}} \right)}^2}}
\end{array}$
