b
\(\begin{array}{l}
at\,t = 1\\
\,{u_x} = 5,\,{u_y} = 5\sqrt 3 \\
{v_y} = 5\sqrt 3  - 10\,\,;\,\,\,{v_x} = 5\\
\tan \,\theta \, =  - 2\left( {2 - \sqrt 3 } \right) \Rightarrow \theta  =  - {30^ \circ }\\
R = \frac{{{v^2}}}{{a \bot }} = \frac{{{{10}^2}}}{{\left( {10\,\cos \,{{30}^ \circ }} \right)}}
\end{array}\)

\(\begin{array}{l}
\,\, = \frac{{10}}{{\sqrt 3 }} \times 2\, = \frac{{20}}{{\sqrt 3 }}m\\
\frac{{{5^2} + {{\left( {10 - 5\sqrt 3 } \right)}^2}}}{{10\,\cos \theta }} = \frac{{200 - 100\sqrt 3 }}{{10 \times 0.965}} = 2.8m
\end{array}\)