title: Mathematica doesn't solve wave equation when given boundary conditions
body: <p>Still new to Mathematica syntax.  When I do:</p>
  
  <pre><code>DSolve[{
    D[u[x, t], {x, 2}] == (1/(v*v))*D[u[x, t], {t, 2}],
    u[0, t] == 0,
    u[l, 0] == 0
    }, u, {x, t}]
  </code></pre>
  
  <p>it just returns what I entered</p>
  
  <pre><code>DSolve[{(u^(2,0))[x,t]==(u^(0,2))[x,t]/v^2,u[0,t]==0,u[l,0]==0},u,{x,t}]
  </code></pre>
  
  <p>However, when I remove the boundary conditions, I get </p>
  
  <pre><code>{{u-&gt;Function[{x,t},C[1][t-(Sqrt[v^2] x)/v^2]+C[2][t+(Sqrt[v^2] x)/v^2]]}}
  </code></pre>
  
  <p>with C[1] and C[2] representing the functions for the boundary conditions.</p>
  
  <p>Anyone know why this is happening?</p>