Solving an equation in multiple variables

asked 2014-11-15 12:08:18 +0200

gelatine1
31 ●2 ●4 ●8

updated 2014-11-15 12:09:09 +0200

I have this equation : sqrt((b-a)^2 + (B-A)^2) == A+B and i would like to have it solved to b-a=+- 2 * sqrt(AB) in sage. Right now I have the following code but it doesn't really output what I want. I just get
A^2 - 2AB + B^2 + a^2 - 2ab + b^2 == A^2 + 2AB + B^2
and I don't know how to make sage solve it further.

my code:

var('a,b,c,A,B,C')
eq1 = (sqrt((b-a)^2 + (B-A)^2) == A+B)
(eq1^2).expand()

Thanks in advance

answered 2014-11-15 14:30:25 +0200

god.one

395 ●7 ●15 ●26

You can try the following with substituting b-a with c

var('a,b,c,A,B,C')
eq1 = (sqrt((c)^2 + (B-A)^2) == A+B)
assume(A>0)
assume(B>0)
solve(eq1,c)

which results in

[c == -2*sqrt(A*B), c == 2*sqrt(A*B)]

But can anybody explain me the results of the computation without the assume statements? For A,B both beeing negative the solver does not do anything and for A being bigger and B being smaller then zero, sage throws an error that maxima needs further inputs (the same as when you're not using assume)

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Comments

If there was no assume, an expression like sqrt(A*B) is not very well defined. The same kind of behavior occurs with simplify_radical.

vdelecroix ( 2014-11-15 14:42:42 +0200 )edit

Thanks, but in this case using different assume scenarios resulting in different results is strange (escpecially in the both is negative case)

god.one ( 2014-11-16 07:03:19 +0200 )edit

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