题目内容：

Mathematica 解常微分方程组问题
Mathematica 输入：
DSolve[{{x1'[t] == k1[t]*(x2[t] - x1[t]),
y1'[t] == k1[t]*(y2[t] - y1[t]),x2'[t] == k2[t]*(x3[t] - x2[t]),
y2'[t] == k2[t]*(y3[t] - y2[t]),x3'[t] == k3[t]*(x4[t] - x3[t]),
y3'[t] == k3[t]*(y4[t] - y3[t]),x4'[t] == k4[t]*(x1[t] - x4[t]),
y4'[t] == k4[t]*(y1[t] - y4[t]),(x1'[t])^2 + (y1'[t])^2 ==
v^2,(x2'[t])^2 + (y2'[t])^2 == v^2,(x3'[t])^2 + (y3'[t])^2 ==
v^2,(x4'[t])^2 + (y4'[t])^2 == v^2},{x1[0] == 0,y1[0] == a,
x2[0] == 0,y2[0] == 0,x3[0] == a,y3[0] == 0,x4[0] == a,
y4[0] == a,k1[0] == v/a,k2[0] == v/a,k3[0] == v/a,
k4[0] == v/a}},{x1[t],y1[t],x2[t],y2[t],x3[t],y3[t],x4[t],
y4[t],k1[t],k2[t],k3[t],k4[t]},t]
输出为：
{}
请问大侠哪里出错了?