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at the mark of 4:23 should it be R12^2+R23^2=R13^2? which would still come out to 2R12^2 but it did confuse me.

Hi merkinthedark,

Thanks for spotting that. Yes, you're quite right that there's a small error in the subscripts, and it should be $r_{12}^2 + r_{23}^2 = r_{13}^2$. I've made a note in the quick answer for other students.

All the best with your studies,
Mr. Dychko