find a root problem

2013 3 26
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syms hr;
Bo=2106192442915567908282078859207/4494983136974228414275670063706112 - (2106192442915567908282078859207*exp(-18446744073709551616/(1147831826363933*(81508471552582434684928/(3187597375937011*hr) + 22182.30127333101796019592417497))))/4494983136974228414275670063706112
hr1=solve(hr-661*230*Bo^0.5,hr)
I found a hr1=2.3145157172213798324089674315726e-40 - 1.8416117030019738856259409912694e-48*i through matlab
But It must be,true answers are 1997.001744 and 0,
I'm waiting your solutions pls Thanks,,

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回答（1 个）

Walter Roberson
Walter Roberson 2013-3-26

0 个投票

You are encountering round=off error. 2106192442915567908282078859207/4494983136974228414275670063706112 is being converted to floating point by MATLAB before the expression reaches MuPAD.
V2106 = sym('2106192442915567908282078859207');
V4494 = sym('4494983136974228414275670063706112');
Bo = V2106/V4494 - (V2106 * exp( sym('-18446744073709551616') / (sym('1147831826363933') * (sym('81508471552582434684928') / (sym('3187597375937011') * hr) + sym('22182.30127333101796019592417497'))))) / V4494;
hr1 = solve( hr - sym('661') * sym('230') * sqrt(Bo), hr)

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emre karakoc
emre karakoc 2013-3-26
Actually the numbers you said not a numbers.These are symbols' value.They change for each iterations in the program. So i can't fix the numbers as a sym.
Also if i do this type,result is 0 but i want to use other result(hr=1997)
Walter Roberson
Walter Roberson 2013-3-27
You did not code them as symbolic values, not here and not in your duplicate question. If you are generating this code, then alter the generation routine to generate symbolic numbers.
Walter Roberson
Walter Roberson 2013-3-27
Try using assume() to add the assumption that hr > 0.
emre karakoc
emre karakoc 2013-3-28
编辑：emre karakoc 2013-3-28
Thanks Walter, i can find '0'.But the answers must be 0 and 1997.After the calculation i can see only 0 as a result. why can't I see the other root?I used assume ()command but it doesn't work. when i use this command, the answer is seen as [empty sym]
Walter Roberson
Walter Roberson 2013-3-28
You are encountering round-off error, and the slope is very very steep near the 1997 solution. The program probably cannot isolate the root. You might need to increase Digits a fair bit (to 40 or so) to find the second root.
I do not have MuPAD, so I cannot test in MuPAD. The symbolic package I use does find the 1997-ish root.

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