Ask Your Question

# wrong variable in solution of an inequality

Hi everybody,

I am new to sage, trying to solve an inequality using sage, i.e:

m,a,d,w,e,c=var('m,a,d,w,e,c')

x_br=1/2*(2*c*d - d*e - d*w)/(2*d*m - e + w)

solve(x_br<=d/2,m)

But what I get is:

[[w == -2*d*m + c], [d == 0], [max(-2*d*m + e, -d*m + c) < w, 0 < d], [-2*d*m + e < w, w < -d*m + c, d < 0, -d*m + e < c], [-d*m + c < w, w < -2*d*m + e, d < 0, c < -d*m + e], [w < min(-d*m + c, -2*d*m + e), 0 < d]]

It is solved based on w instead of m. I tried different variables but the result is always the same. What should I do to have solution based on x?

Thank you in advance.

edit retag close merge delete

## 1 Answer

Sort by » oldest newest most voted

The problem here might be that Sage doesn't know whether your variables are positive or negative. If you don't make more assumptions there may be no way to solve the inequality symbolically.

It looks like in your inequality, the solution would strongly depend on whether (2*d*m - e + w) is positive or negative, and also whether d is positive or negative.

Try looking at the documentation for the function solve_ineq which uses Maxima to solve single inequalities in one variable or systems of inequalities in several variables.

more

## Comments

Thank you. But actually there is no assumptions about (2*d*m - e + w) in my problem. The only assumption is d>0 which when I apply it, it does not make any changes in the solution. Although I solved the problem by hand, I am so curious to know how to achieve desired arrangement in variables which may help me through remaining of my work.

## Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

## Stats

Asked: 2011-12-03 10:37:53 +0200

Seen: 138 times

Last updated: Dec 03 '11