Giải thích các bước giải:
Ta có:
$(x_1a-y_1b)\cdot 2n+(x_2a-y_2b)\cdot 2n+(x_3a-y_3b)\cdot 2n+...+(x_ma-y_mb)\cdot 2n<0$
$\to 2n(x_1a-y_1b+x_2a-y_2b+x_3a-y_3b+...+x_ma-y_mb)<0$
$\to (x_1a-y_1b+x_2a-y_2b+x_3a-y_3b+...+x_ma-y_mb)<0$ vì $n\in N^*\to n>0$
$\to x_1a+x_2a+x_3a+...+x_ma<y_1b+y_2b+y_3b+...+y_mb$
$\to a(x_1+x_2+x_3+...+x_m)<b(y_1+y_2+y_3+...+y_m)$
$\to \dfrac{x_1+x_2+x_3+...+x_m}{y_1+y_2+y_3+...+y_m}<\dfrac{b}{a}$