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, , . , - , , , , , , . , . - , - , , , .
, . , ? , , , , , , , ( , ); , , .
, . . 20.14a, . : , , - "" " " (. 20.14a). " !" - . , . 20.14b. , a < b z(a) > z(b).
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z-
. z- . () , z- - , z . z , , , z-. , , , , , z- z. , . , z(x, ).
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- . , . .
- .
z-.
1. , z- - zmin.
2. :
ax + by + cz + d = 0 - .
(x1, y1, z1)
(x2, y2, z2)
...
(xn, yn, zn)
a = Δ(yi - yi+1) * (zi + zi+1)
b = Δ(zi - zi+1) * (xi + xi+1)
c = Δ(xi - xi+1) * (yi + yi+1)
z = -(ax + by + d)/c.
.
z - z' = -(ax1 + d)/c + (ax + d)/c = a(x - x1)/c
z1 = z - (a/c)Δx, Δx = 1,
z1 = z - (a/c), z1 - , z - .
3. (x, y) z(x, y) - .
4. z z, .
z z, , z z.