Date: Sat, 29 Nov 1997 08:27:02 -0500
From: Alex Bogomolny
Dear Denis:
Your formula is good but the question is ill conceived. There is no standard formula for the height in a triangle. Mathematics would be boring if there were standard formulas. There is a great variety of them and it's up to you to choose the best suited for your purposes, or come up with a new one.
I can tell you this. Your formula is correct regardless of how you arrived at it. It also follows from the cosine law. This would be a direct verification. There is an indirect one:
Your formula is not symmetric with respect to b and c although in reality (at a glance) AD relates to b and c in the same manner and differently form its relation to a. Therefore, the formula obtained from yours by swapping b and c must also be correct. See that this is so
AD2 = sqrt(b2 - ((a2 + c2 - b2) / 2a)2).
Of the formulas I know, there is one most reminiscent of yours. It's the one that leads to (or is a consequence of) Heron's formula for the area of a triangle:
AD2 = 4p(p-a)(p-b)(p-c)/a2,
where p is the semiperimeter: p = (a+b+c)/2.
Glad you enjoy tinkering with math.
Sincerely,
Alexander Bogomolny
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