2
. , , . x y, z.
, , (x, y) D , f (x, y) f (x, y).
D .
: x, y M , z = f (x, y) z = f (M). x, y M.
: x z = f (x, y) M 0(x 0, y 0)
,
. :
; ; .
x z = f (x, y), x y.
y z = f (x, y) M 0(x 0, y 0):
= .
. , x z = f (x, y) y , y x.
. 1. .
2.
:
.
. x y. zxx ¢¢, zyy¢¢, zxy¢¢ . ; . .
M 0(x 0, y 0) () z = f (x,y), M 0, M (x,y) f (x,y)< f (x 0 ,y 0) (f (x,y)> f (x 0 ,y 0)).
.
: ( - ), .
: , .
, , ( ).
: z = xy; zx ¢ = y; zy ¢ = x; zx ¢(0,0) = 0; zy ¢(0,0) = 0.
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(0,0) 0. (0,0) , z = 0, , z (x,y) > 0 ( , ), , z (x,y) < 0 ( , ).
, , :
zx ¢(x 0 ,y 0) = 0 zy ¢(x 0 ,y 0) = 0, z (x 0 ,y 0). : A = zxx ¢¢(x 0 ,y 0); B = zxy ¢¢(x 0 ,y 0); C = zyy ¢¢(x 0 ,y 0); D = AC - B 2.
, D < 0, (x 0 ,y 0) .
D > 0, (x 0 ,y 0) z, A > 0, , A < 0, .
D = 0, , . .