The Gemara explains why we don’t add another bracha to Havineinu on motzei Shabbos (for havdala) since Havineinu was established specifically as a ‘seven bracha unit’ and not eight. If Havineinu assumes a new identity as a ‘seven bracha tefilla’ then why doesn’t the Gemara require a source corresponding to this number as the Gemara sought regarding the various types of shemone esrei (i.e. 18 of weekdays, 7 of Shabbos etc.)?
The Gemara tells us that one is not meant to make mention of personal needs during the first and last three brachos of shemone esrei which are dedicated to the praise of hashem. Tosfos points out that this is only said regarding one’s own personal matters, however one may pray for public needs. If the reason for this halacha is that we must first open our tefilla specifically with praise, what difference is there between personal needs or public needs?