Britney Femme Fatale Tour- Live Vocals?

Hey so as we know Britney has started her world tour and has wowed
fans unbelievably with her two first shows. She appears happy, fit
dancing and singing LIVE!
However, i remain skeptical.
I compared the two performances and i do not believer she is singing
live, rather she is using very clever pre-recorded vocals.
What are your thoughts?

compare these-

'Hold it against me'
san jose:
http://www.youtube.com/watch?v=Tz_s8ef-sfIfeature=mfu_in_orderlist=UL

sacramento: http://www.youtube.com/watch?v=KX2lZxKciCc

'3'
san jose: http://www.youtube.com/watch?v=K7ZVD0BExlI
sacramento: http://www.youtube.com/watch?v=99fgiqnWkjc

'trouble for me'
san jose:
http://www.youtube.com/watch?v=UWMMg5L5m-ofeature=mfu_in_orderlist=UL

sacramento: http://www.youtube.com/watch?v=z9YrQBQahLM

If any one can help or has been to the concert that would be great :)
Let's have a discussion :)