Giải thích các bước giải:
Ta có:
\[\begin{array}{l}
a,\\
f\left( {{x_1} + {x_2}} \right) = k\left( {{x_1} + {x_2}} \right) = k{x_1} + k{x_2} = f\left( {{x_1}} \right) + f\left( {{x_2}} \right)\\
b,\\
f\left( {kx} \right) = a.\left( {kx} \right) = k.\left( {ax} \right) = k.f\left( x \right)\\
c,\\
f\left( {{x_1}} \right).f\left( {{x_2}} \right) = a{x_1}.a{x_2} = a.\left( {a{x_1}{x_2}} \right) = a.f\left( {{x_1}.{x_2}} \right)
\end{array}\]