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计算

$$
D=\left|\begin{array}{llll}
x_1 y_1 & x_1 y_2 & x_1 y_3 & x_1 y_4 \\
x_1 y_2 & x_2 y_2 & x_2 y_3 & x_2 y_4 \\
x_1 y_3 & x_2 y_3 & x_3 y_3 & x_3 y_4 \\
x_1 y_4 & x_2 y_4 & x_3 y_4 & x_4 y_4
\end{array}\right|
$$
                        
不再提醒