求解偏微分方程组为什么输出和输入显示一样
 

输入：
Clear[eqn, sol, u, v, x, y];
eqn = {u[x, y] D[u[x, y], x] + v[x, y] D[v[x, y], y] == 0, v[x, y] D[u[x, y], y] + u[x, y] D[v[x, y], x] == 0};
sol = DSolve[eqn, {u[x, y], v[x, y]}, {x, y}]

输出：
DSolve[{u[x,y] (u^(1,0))[x,y]+v[x,y] (v^(0,1))[x,y]==0,(u^(0,1))[x,y] v[x,y]+u[x,y] (v^(1,0))[x,y]==0},{u[x,y],v[x,y]},{x,y}]

请问各位大虾，为什么我的输出结果还是DSolve形式？