My answer to a question on Quora: My high school math teacher often made rudimentary mistakes in her equations. What does that say about her?
This could be turned into a good learning experience for you: watch your teacher and try instantly identify her mistakes and correct them – for yourself. I had this experience in my school days, and remember it fondly. It really helps to start mastering mathematics.
It is up to you to decide whether to try to correct your teacher in front of the class, but remember, it could be very cruel to her, you may regret that later. It would be much more useful to quietly provide your classmates with correct solutions.
And maths teachers in schools are frequently overworked. And what do you know about her home life? She could simply suffer from long term sleep deprivation. Especially in the present crazy times…