Hi, thank you for you answer. Sorry, but the r[i] and r[i+1] coordinates are column vectors, not row vectors. The forum parses the brackets used for column vectors, so I (stupidly) changed them to the brackets for row vectors, without mentioning it. Sorry. I do get the same result as you when I transform them to row vectors, but that would yield an incorrect equation.

Hi, thank you for you answer. Sorry, but the r[i] and r[i+1] coordinates are column vectors, not row vectors. The forum parses the brackets used for column vectors, so I (stupidly) changed them to the brackets for row vectors, without mentioning it. Sorry. I do get the same result as you when I transform them to row vectors, but that would yield an incorrect equation.