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plot z = sqrt(1-(x^2+y^2)), x = -1..1, y = -1..1
plot z = sin(x)+cos(2*y), x = -3..3, y = -3..3
plot z = x*y^2, x = -3..3, y = -3..3
plot z = min(sqrt(x^2+y^2),4), x=-4..4, y=-4..4
plot z = min(sqrt(x^2+y^2),4.05), x=-4..4, y=0..4
d/dy d/dx x*y^2
plot z = sin(x*y)
plot z = -x^3+y/1e9, -3 < x <3, -3 < y < 3
  (y/1e9 e' trascurabile; serve per far capire z e' di 2 variabili)
plot z =( -18*x)/(2+x^2+y^2), -10 < x < 10, -10 < y < 10
plot z = 1 / (1+(1-sqrt(x^2+y^2))^2), -4 < x < 4, -4 < y < 4
Hessian (sin(x)+cos(2*y))

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