# Revision history [back]

When you write:

dqdt[0].subs({qs[i]:z[i] for i in range(0,N)})


this returns another object, it does not change dqdt.

So, instead, you can write

dqdt[0] = dqdt[0].subs({qs[i]:z[i] for i in range(0,N)})


When you write:

dqdt[0].subs({qs[i]:z[i] for i in range(0,N)})


this returns another object, it does not change dqdt.

So, instead, you can writewrite:

dqdt[0] = dqdt[0].subs({qs[i]:z[i] for i in range(0,N)})