y
f(x, y) = arcsin "f f
x
uxx + uxy = 0
¾ = y, · = y - x.
1 1 1
f(x, y, z) = x + y + z + + +
x y z
a > 0

x2
xyds Å‚ + y2 = 1
Å‚ 4

2dx+xdy
Å‚ y = x2 (0, 0) (1, 1)
Å‚ x2+y2
f(x, y) = ln(x - y2) "f f
uxx + uxy - 2uyy = 0
¾ = x + y, · = 2x - y.
f(x, y, z) = x + y + z - ln(xyz), (x, y, z > 0)
a > 0

y2
xyds Å‚ x2 + = 1
Å‚ 9

ex(dx + ydy) Å‚ y2 = x (0, 0) (1, 1)
Å‚