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Wrong answer on inequality problem

asked 2015-10-17 11:15:53 -0500

leothan gravatar image

updated 2015-10-17 14:04:50 -0500

The problem I'm having is the solution I get on sage when solving the following inequality

[(4*x+5)/(x^2)>=4/(x+5)]

It gives me the wrong answer

([x < -5], [x >= -1])

the answer should be

(x<-5),(-1<=x<0),(0<x<oo).

The answer can be checked at here at wolframalpha. I'm still new to sage and in the learning process I don't know if I might be doing something wrong.

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answered 2015-10-17 14:06:50 -0500

updated 2015-10-17 14:09:30 -0500

The problem seems to be that Sage makes sense of "+infinity > finite number" situations. Even in the following where both sides of the inequality are infinite

sage: A = (4*x+5)/(x^2)
sage: B = 4/(x+5)
sage: solve(A >= B, x)
[[x < -5], [x >= -1]]
sage: solve(A/x^2 >= B/x^2, x)
[[x < -5], [x >= -1]]

Vincent

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Asked: 2015-10-17 11:15:53 -0500

Seen: 53 times

Last updated: Oct 17 '15