Hi, I want to solve following system of differential equations

sola = DSolve[{x5'[t] == -b, x1'[t] == x2[t], 
   x2'[t] == (k*b*Cos[fi])/x5[t], x3'[t] == x4[t], 
   x4'[t] == k*b*Sin[fi]/x5[t] - g, x1[0] == 0, x2[0] == 0, 
   x3[0] == 0, x4[0] == 0, x5[0] == m0}, {x1[t], x2[t], x3[t], x4[t], 
   x5[t]}, t]

Gives following output:

{{x5[t] -> m0 - b t, 
  x2[t] -> k Cos[fi] Log[-m0] - k Cos[fi] Log[-m0 + b t], 
  x1[t] -> (
   b k t Cos[fi] - k m0 Cos[fi] Log[-m0] + b k t Cos[fi] Log[-m0] + 
    k m0 Cos[fi] Log[-m0 + b t] - b k t Cos[fi] Log[-m0 + b t])/b, 
  x4[t] -> -g t + k Log[-m0] Sin[fi] - k Log[-m0 + b t] Sin[fi], 
  x3[t] -> (-b g t^2 + 2 b k t Sin[fi] - 2 k m0 Log[-m0] Sin[fi] + 
    2 b k t Log[-m0] Sin[fi] + 2 k m0 Log[-m0 + b t] Sin[fi] - 
    2 b k t Log[-m0 + b t] Sin[fi])/(2 b)}}

but there is a mistake in the Log[-m0], there should be a +m0. How can I thread this?