Dear Politico, I don't know what the Khan Academy is but it sounds like might be related to martial arts and so I think maybe avoiding any type of proper noun will be best.
I would just go meta on this and say that you have taught yourself calculus through the use of free resources.
I generally like to be specific, but I think in this situation it might distract the reader away from the main point. Is this something you're writing in your essay is and does this point to a larger thing that you're talking about? Because I don't want you to deck an entire essay against this very issue. I
If it is related to how you take initiative then that's fine but if you're trying to tell them you're good at math, that might be a little bit of a waste.
If you have any concerns about their perceptions of your their perceptions of your Quant skills, I would just do MBA math. It's really a great course to take anyways, even outside of the role it might play in admissions.
That's really the gold standard for all of this, you can send your transcripts to all the schools directly from Mba Math.
Farrell Dyan Hehn, MBA
Admissions Consultant & Verbal Strategist MBAPrepCoach.com