Thực hiện phép tính (tính hợp lí nếu có thể): j) − 10/21 ⋅ [ 9/15 + ( 3/5 )^2 ] .

j) \( - \frac{{10}}{{21}} \cdot \left[ {\frac{9}{{15}} + {{\left( {\frac{3}{5}} \right)}^2}} \right]\)

\( = - \frac{{10}}{{21}} \cdot \left[ {\frac{3}{5} + \frac{9}{{25}}} \right]\)

\( = \frac{{ - 10}}{{21}} \cdot \left[ {\frac{{15}}{{25}} + \frac{9}{{25}}} \right]\)

\( = \frac{{ - 10}}{{21}} \cdot \frac{{24}}{{25}}\)

\( = \frac{{ - 10 \cdot 24}}{{21 \cdot 25}} = \frac{{ - 16}}{{35}}.\)