I dont know where to start
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Altitude -- from Wolfram MathWorld
In most cases the altitude of the triangle is inside the triangle, like that image example
AB = c, BC = a, AC = b 24 b = 56 c = 21 a = twice area ABC solve for b, c in terms of a and replace in $\displaystyle a^2+c^2-2 a c \cos B=b^2$ we get $\displaystyle \cos B=1/2$