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Thanks for your answer. But how do I know the value of t for x[t] == 0? Also I tried a plot and it looks like x[t] continues beyond x=0 which is not what I'd excpect ode1 = {x'[t] == -1, x[0] == 1, y'[t] + y[t] x[t]^2 == 100, y[0] ==...