I have to solve the task about mathematical pendulum.

And I have formula:

x''(t)+g/k*sin(x(t))=0, where x - angle of pendulum.

And for solve this we have

x(t=0) = 0, x'(t=0) = a

But when I write in mathematica:

DSolve[{x''[t] + g/l*Sin[x[t]] == 0, x'[0] == 0, x[0] == a}, x[t], t]

I have three errors:

Solve::ifun: Inverse functions are being used by Solve, so some solutions may not be found; use Reduce for complete solution information. >>

DSolve::bvfail: For some branches of the general solution, unable to solve the conditions. >>

DSolve::bvfail: For some branches of the general solution, unable to solve the conditions. >>

I have spend several hours, but with no result...

Please, help me to understand - what happens...