q[x1_, x2_] :=
-x2+Tanh[x2]-Sin[x1];
NDSolve[{x1'[t] == x2[t], x2'[t] == q[x1[t],x2[t]],
x1[0] == 0.01, x2[0] == 0.01}, {x1[t], x2[t]}, {t, 0, 100}];
A[x1_,x2_]:= Cosh[x2];
NDSolve[{x1'[t] == x2[t], x2'[t] == q[x1[t],x2[t]]+A[x1[t],x2[t]]*b[t],
x[0] == 0.01, v[0] == 0.01}, {x[t], v[t]}, {t, 0, 10}];
