[问题] DEplot运行后的error

[maomao_hhu]

因为是非线性微分方程组的，所以老师说用deplot算一个six equations each with second order的微分方程组，
每个方程类似于：
de1 := diff(x1(t), t, t)+(1/2)*(sqrt((x3(t)-x1(t))^2+(y3(t)-y1(t))^2)-2)*(-2*x3(t)+2*x1(t))/sqrt((x3(t)-x1(t))^2+(y3(t)-y1(t))^2)+(1/2)*(sqrt((x2(t)-x1(t))^2+(y2(t)-y1(t))^2)-2)*(-2*x2(t)+2*x1(t))/sqrt((x2(t)-x1(t))^2+(y2(t)-y1(t))^2) = diff(x2(t), t)-2*(diff(x1(t), t))+diff(x3(t), t)
组成系统：
sys := {de1, de2, de3, de4, de5, de6}

最后输入命令：
DEplot([sys], [x1(t), x2(t), x3(t), y1(t), y2(t), y3(t)], t = 0 .. 100, [[x1(0) = 1, x2(0) = 3, x3(0) = 2, y1(0) = 1, y2(0) = 1, y3(0) = 2.73, (D(x1))(0) = 1, (D(x2))(0) = 0, (D(x3))(0) = 0, (D(y1))(0) = 0, (D(y2))(0) = 0, (D(y3))(0) = 0]])

结果出现：
Error, (in DEtools/DEplot) system must have same number of dependent variables as DE's.
我觉得变量没有问题啊，求各位高手指教。