Cho hai điểm\(A\left( {2;0} \right);B\left( {1;2}...

C

- Hướng dẫn giải

\(\begin{array}{l} N(x;\,\,y)\\ NA = 2NB\\ \Rightarrow {\left( {x - 2} \right)^2} + {y^2} = 4\left( {{{\left( {x - 1} \right)}^2} + {{\left( {y - 2} \right)}^2}} \right)\\ \Leftrightarrow 3{x^2} - 4x + 3{y^2} - 16y + 16 = 0\\ \Leftrightarrow {\left( {x - \frac{2}{3}} \right)^2} + {\left( {y - \frac{8}{3}} \right)^2} = \frac{{20}}{9}\\ a + b + {R^2} = \frac{2}{3} + \frac{8}{3} + \frac{{20}}{9} = \frac{{50}}{9}\\\approx5,5 \end{array}\)