问题

I am trying to solve 2 simultaneous equations, which lead to two sets of x-y values. I only want the x-y set that has a positive x.

我试图解决两个同时出现的方程，这将导致两组x-y值。我只想要x为正的那组x-y值。

I set my assumption in x when defining it to be positive=True. Yet when I get my x-y solutions, I still only get the [-1.41421356237310], [2.00000000000000] set when I want [1.41421356237310], [2.00000000000000] set. It appears that my assumption to make x positive is not working, although the print((x).is_positive() returns True. Why is my assumption not working, and how do I force nsolve to assume that x must be positive? The code can be found below.

我在定义x时将它的假设设置为positive=True。然而，当我获得x-y的解时，我仍然只得到[-1.41421356237310], [2.00000000000000]这组值，而我想要的是[1.41421356237310], [2.00000000000000]这组值。看来我的假设x为正并没有起作用，尽管print((x).is_positive)返回True。为什么我的假设不起作用，我如何强制nsolve假定x必须为正？下面是代码。

import sympy as sym

x = sym.symbols('x', positive=True)
y = sym.symbols('y', positive=True)
eq1 = sym.Eq(y, x**2)
eq2 = sym.Eq(y, 2)
result = sym.nsolve((eq1, eq2), (x, y), (-1, -5))
print(result)
print((x).is_positive)

import sympy as sym

x = sym.symbols('x', positive= True)
y= sym.symbols('y', positive= True)
eq1= sym.Eq(y,x**2)
eq2= sym.Eq(y,2)
result= sym.nsolve((eq1, eq2), (x,y),(-1,-5))
print(result)
print((x).is_positive)

答案1

得分: 2

你应该使用 solve 而不是 nsolve，因为只有前者会使用变量的假设：

...
result = sym.solve((eq1, eq2), (x, y))
print(result)

如预期结果所示：

[(sqrt(2), 2)]

（如果删除 x 上的 positive=True 假设，你会得到 [(sqrt(2), 2), (-sqrt(2), 2)]）

...
result = sym.solve((eq1, eq2), (x, y))
print(result)

[(sqrt(2), 2)]