Imaris
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90% of my linear algebra notebook contains crossed-out solutions to the simplest problems. I hate this damn arithmetic.
Added 1 minute later
It's impossible. I skipped two linear algebra lectures, and now it's hard to catch up. It seems easy, linear algebra is just a piece of cake, but it's not. The textbooks are crap, the internet is crap, my classmates' brains are crap, so you have to sit and figure everything out yourself, create theorems and derive consequences from them.
Added 1 minute later
It's impossible. I skipped two linear algebra lectures, and now it's hard to catch up. It seems easy, linear algebra is just a piece of cake, but it's not. The textbooks are crap, the internet is crap, my classmates' brains are crap, so you have to sit and figure everything out yourself, create theorems and derive consequences from them.