Hello again,
I've been doing a project where I use ARI(1,1) with outliers as the model for forecasting.
I'm planning to do the forecast manually through Ms. Excel but I find a problem.
So, here's the model.

The problem is I don't know what will happen to the 1/(1-B) if the I(49) valued as 1.
Thankyou so much for your consideration to help me solve this problem.
Best regards.