just Bayes rule apply now we had some assumptions that the measurement at time T given XT does not depend on anything else so we just simplify accordingly okay so we've not yet gone from XT minus 1 to XT we've incorporated the last step the measurement can we now do the transition from XT minus 1 to XT well let's think about it how do we go from XT minus 1 to XT well xt- one I've already have it we're multiplying the dynamics model to get the joint and then sum out XT minus one okay that's exactly what we're going to do we