问题
Is there a way to specify the domain for a function in wolfram alpha? I have a function which I want to plot. I want to restrict the function f(x,y) = xy(3-x-y) to x>=0, y>=3 and y <=3-x
回答1:
You can specify boundaries of the function parameters in Wolfram Alpha. They are more of a hint to the plotting library how much to scale the plot than exact boundaries, so in your case y is not calculated only with a value of 3. Although the boundaries in your case do not make much sense, as noted by @Sjoerd, this answer could be useful for setting sensible boundaries.
To plot your function, write the following in Wolfram Alpha:
f(x,y) = xy(3-x-y) for x>=0, y=3..3-x
It works better with boundaries set around zero and univariate functions:
plot y^2 cos(x), x=-6..6, y=-2..2
plot f(x) = -8/x + sqrt(7/x^2+1/x+1) for x=1..50
Reference: Wolfram Alpha Blog
回答2:
Your restrictions make no sense. For x>=0 it follows that y<=3-x equals y<=3, but you also have y>=3, so this means that you want a plot for y=3. So, what you want to plot is apparently f(x)= -3x^2.
Other than that, I don't think WA currently allows a RegionFunction as in Mathematica
来源：https://stackoverflow.com/questions/5087461/how-to-specify-domain-of-a-function-in-wolfram-alpha