АМ- медиана, т.е. M - середина стороны BC.
X=(Xb+Xc)/2 = (0+3)/2 = 3/2
Y=(Yb+Yc)/2 = (2+1)/2 = 3/2
Z=(Zb+Zc)/2 = (2+0)/2 = 1
M(3/2, 3/2, 1)
AM = (3/2-(-1), 3/2-0, 1-(-4)) = (5/2, 3/2, 5)
|AM| = sqrt( (5/2)^2 + (3/2)^2 + 5^2) = sqrt(25/4 + 9/4 + 25) = sqrt(34/4 + 25) = sqrt(134/4) = sqrt(134)/2