Khan Academy
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I don't know why everyone is hating on…
I don't know why everyone is hating on it so much. It's free for God's sake. They provide free education for people who struggle to grasp hard concepts, mostly in the STEM field. My only advice on using it is DO NOT CARE about its grading system. What is important is that you learn. Also if you have anger issues, I advise you to just take a breather.