Solving a linear equation

asked 2011-08-29 16:12:56 +0200

tmaxara
105 ●4 ●6 ●10

updated 2011-08-29 16:40:00 +0200

kcrisman
12222 ●41 ●135 ●254

Hello,

I have a problem with the solution of a linear equation given by sage.

sage: a,b,c,d=var('a,b,c,d')
sage: v(t)=a*t^3+b*t^2+c*t+d
sage: v1(t)=diff(v(t),t)
sage: g1=v(0)==0
sage: g2=v(5)==1
sage: g3=v1(5)==0
sage: g4=v(1.5)==0.2
sage: solve([g1,g2,g3,g3],[a,b,c,d])
[[a == r1, b == -10*r1 - 1/25, c == 25*r1 + 2/5, d == 0]]

The right solution is a=-62/3675, b=473/3675, c=-16/735, d=0!

What's wrong with my implementation in sage?

Thomas

By the way: How can I realize the typical pre-formatted "sage:... look" in my postings??

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I've edited it to show you how to get that wonderful pre-formatted look - just click "edit" to see how it's done!

kcrisman ( 2011-08-29 16:41:51 +0200 )edit

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answered 2011-08-29 16:46:47 +0200

kcrisman
12222 ●41 ●135 ●254

Your answer is just one of the ones for this.

sage: r1=-62/3675
sage: -10*r1-1/25
473/3675
sage: 25*r1+2/5
-16/735

Maxima returns a solve like this when there is a parameter. In this case, one parameter, r1, which can be any real number. Are you sure your matrix has non-zero determinant?

Or did you intend

sage: solve([g1,g2,g3,g4],[a,b,c,d])

instead of

sage: solve([g1,g2,g3,g3],[a,b,c,d])

Actually, I think this is the real issue - typos trip us all up :(

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Or did you intend sage: solve([g1,g2,g3,g4],[a,b,c,d]) instead of...

Yes!! Yesterday I sat an hour ...and now I have made a spelling error!

Thank you Thomas

tmaxara ( 2011-08-30 05:07:46 +0200 )edit

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