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How to say go-al Yisroel before Shacharis Amida
According to the Shulchan Aruch (66:7), one doesn't say "amen" after the chazan says "Ga'al Yisra'el." The Rama says that you do. The Mishnah Brurah (35) says that the proper thing to do is to fulfill all opinions and say the conclusion of the blessing together with the chazan, or to say the words "י"י שפתי תפתח" before the chazan concludes.
I have seen that the practice is for the chazan to say "Ga'al Yisra'el" silently, which seems to be in response to this problem. Is there any source or reason to use this solution other than the two the Mishnah Brurah quotes?