Solve eqn and get real solution

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ly
ly 2015-10-8
编辑： ly 2015-10-8
I want to solve an eqn and get the real solution. This is the code:
...
s=solve(R_);
...
And this is solution: s =
0.3971412063699492853956558622094
-0.8763850946630804323385057262195
- 0.13537805585343442652857506799495 - 0.83663803641655759940578883730279*i
- 0.13537805585343442652857506799495 + 0.83663803641655759940578883730279*i
As you can see "s" has two solutions are real (0.3971 and -0.87638) and two imaginary (with i). The real solution is needed. Which command can be used in this case? Give the example is helpful.

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回答（2 个）

Torsten
Torsten 2015-10-8
s=solve(R_,s,'Real',true);
Best wishes
Torsten.
  1 个评论
ly
ly 2015-10-8
编辑：ly 2015-10-8
I see it is not working. I use MatLab R2011b
This is the code: y is function, s is variable
syms s
y=-(2*((622930548819969075*s^4)/147573952589676412928 - (45369884049352485*s^3)/576460752303423488 + (69272938975695885*s^2)/144115188075855872 - (2396595835289415*s)/2251799813685248 + 2755552026909489/4503599627370496)*((124586109763993815*s^5)/147573952589676412928 - (45369884049352485*s^4)/2305843009213693952 + (23090979658565295*s^3)/144115188075855872 - (2396595835289415*s^2)/4503599627370496 + (2755552026909489*s)/4503599627370496 - 8557508135623347/36028797018963968) + 2*((10728642163125225*s^3)/288230376151711744 - (938834444243547705*s^4)/295147905179352825856 - (72312578171060145*s^2)/576460752303423488 + (3614047914544827*s)/36028797018963968 + 17054950948113267/1152921504606846976)*((10728642163125225*s^4)/1152921504606846976 - (187766888848709541*s^5)/295147905179352825856 - (24104192723686715*s^3)/576460752303423488 + (3614047914544827*s^2)/72057594037927936 + (17054950948113267*s)/1152921504606846976 - 1819839641949093/72057594037927936))/(2*(((17054950948113267*s)/1152921504606846976 + (3614047914544827*s^2)/72057594037927936 - (24104192723686715*s^3)/576460752303423488 + (10728642163125225*s^4)/1152921504606846976 - (187766888848709541*s^5)/295147905179352825856 - 1819839641949093/72057594037927936)^2 + ((2755552026909489*s)/4503599627370496 - (2396595835289415*s^2)/4503599627370496 + (23090979658565295*s^3)/144115188075855872 - (45369884049352485*s^4)/2305843009213693952 + (124586109763993815*s^5)/147573952589676412928 - 8557508135623347/36028797018963968)^2)^(3/2))
m=solve(y,s)
mm=solve(y,s,'Real',true)
This is result:
y =
-(((1341080270390653*s^3)/18014398509481984 - (1833661023913179*s^4)/288230376151711744 - (4519536135691259*s^2)/18014398509481984 + (3614047914544827*s)/18014398509481984 + 4263737737028317/144115188075855872)*((1341080270390653*s^4)/144115188075855872 - (5867715276522173*s^5)/9223372036854775808 - (6026048180921679*s^3)/144115188075855872 + (3614047914544827*s^2)/72057594037927936 + (4263737737028317*s)/288230376151711744 - 1819839641949093/72057594037927936) + ((608330614082001*s^4)/72057594037927936 - (5671235506169061*s^3)/36028797018963968 + (4329558685980993*s^2)/4503599627370496 - (2396595835289415*s)/1125899906842624 + 2755552026909489/2251799813685248)*((7786631860249613*s^5)/9223372036854775808 - (5671235506169061*s^4)/288230376151711744 + (1443186228660331*s^3)/9007199254740992 - (2396595835289415*s^2)/4503599627370496 + (2755552026909489*s)/4503599627370496 - 8557508135623347/36028797018963968))/(2*(((2755552026909489*s)/4503599627370496 - (2396595835289415*s^2)/4503599627370496 + (1443186228660331*s^3)/9007199254740992 - (5671235506169061*s^4)/288230376151711744 + (7786631860249613*s^5)/9223372036854775808 - 8557508135623347/36028797018963968)^2 + ((4263737737028317*s)/288230376151711744 + (3614047914544827*s^2)/72057594037927936 - (6026048180921679*s^3)/144115188075855872 + (1341080270390653*s^4)/144115188075855872 - (5867715276522173*s^5)/9223372036854775808 - 1819839641949093/72057594037927936)^2)^(3/2))
m =
0.85586995465205551704383696315741
3.3845549086104546062118333757515
6.0644608074070649395928866885874
0.86774544065327612268034185895668 - 0.28338346625708125900409850345349*i
0.86774544065327612268034185895668 + 0.28338346625708125900409850345349*i
5.9090579197766355323813767511608 - 3.2214680329909146257449746295832*i
5.9090579197766355323813767511608 + 3.2214680329909146257449746295832*i
6.2175092150938920522167229879549 + 0.81988112353725460786890788489671*i
6.2175092150938920522167229879549 - 0.81988112353725460786890788489671*i
mm =
0.85586995465205551704383696315741
3.3845549086104546062118333757515
6.0644608074070649395928866885874
0.86774544065327612268034185895668 - 0.28338346625708125900409850345349*i
0.86774544065327612268034185895668 + 0.28338346625708125900409850345349*i
5.9090579197766355323813767511608 - 3.2214680329909146257449746295832*i
5.9090579197766355323813767511608 + 3.2214680329909146257449746295832*i
6.2175092150938920522167229879549 + 0.81988112353725460786890788489671*i
6.2175092150938920522167229879549 - 0.81988112353725460786890788489671*i
As you can see both "m" and "mm" solution have imaginary parts "i".
Can you check again?

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Torsten
Torsten 2015-10-8
syms s
y=-(2*((622930548819969075*s^4)/147573952589676412928 - (45369884049352485*s^3)/576460752303423488 + (69272938975695885*s^2)/144115188075855872 - (2396595835289415*s)/2251799813685248 + 2755552026909489/4503599627370496)*((124586109763993815*s^5)/147573952589676412928 - (45369884049352485*s^4)/2305843009213693952 + (23090979658565295*s^3)/144115188075855872 - (2396595835289415*s^2)/4503599627370496 + (2755552026909489*s)/4503599627370496 - 8557508135623347/36028797018963968) + 2*((10728642163125225*s^3)/288230376151711744 - (938834444243547705*s^4)/295147905179352825856 - (72312578171060145*s^2)/576460752303423488 + (3614047914544827*s)/36028797018963968 + 17054950948113267/1152921504606846976)*((10728642163125225*s^4)/1152921504606846976 - (187766888848709541*s^5)/295147905179352825856 - (24104192723686715*s^3)/576460752303423488 + (3614047914544827*s^2)/72057594037927936 + (17054950948113267*s)/1152921504606846976 - 1819839641949093/72057594037927936))/(2*(((17054950948113267*s)/1152921504606846976 + (3614047914544827*s^2)/72057594037927936 - (24104192723686715*s^3)/576460752303423488 + (10728642163125225*s^4)/1152921504606846976 - (187766888848709541*s^5)/295147905179352825856 - 1819839641949093/72057594037927936)^2 + ((2755552026909489*s)/4503599627370496 - (2396595835289415*s^2)/4503599627370496 + (23090979658565295*s^3)/144115188075855872 - (45369884049352485*s^4)/2305843009213693952 + (124586109763993815*s^5)/147573952589676412928 - 8557508135623347/36028797018963968)^2)^(3/2))
m=solve(y,s)
ix=arrayfun(@isreal,double(m));
m=m(ix);
disp(m);
Best wishes
Torsten.
  1 个评论
ly
ly 2015-10-8
编辑：ly 2015-10-8
Your code is run and this is the result in my MatLab:
y =
-(((1341080270390653*s^3)/18014398509481984 - (1833661023913179*s^4)/288230376151711744 - (4519536135691259*s^2)/18014398509481984 + (3614047914544827*s)/18014398509481984 + 4263737737028317/144115188075855872)*((1341080270390653*s^4)/144115188075855872 - (5867715276522173*s^5)/9223372036854775808 - (6026048180921679*s^3)/144115188075855872 + (3614047914544827*s^2)/72057594037927936 + (4263737737028317*s)/288230376151711744 - 1819839641949093/72057594037927936) + ((608330614082001*s^4)/72057594037927936 - (5671235506169061*s^3)/36028797018963968 + (4329558685980993*s^2)/4503599627370496 - (2396595835289415*s)/1125899906842624 + 2755552026909489/2251799813685248)*((7786631860249613*s^5)/9223372036854775808 - (5671235506169061*s^4)/288230376151711744 + (1443186228660331*s^3)/9007199254740992 - (2396595835289415*s^2)/4503599627370496 + (2755552026909489*s)/4503599627370496 - 8557508135623347/36028797018963968))/(2*(((2755552026909489*s)/4503599627370496 - (2396595835289415*s^2)/4503599627370496 + (1443186228660331*s^3)/9007199254740992 - (5671235506169061*s^4)/288230376151711744 + (7786631860249613*s^5)/9223372036854775808 - 8557508135623347/36028797018963968)^2 + ((4263737737028317*s)/288230376151711744 + (3614047914544827*s^2)/72057594037927936 - (6026048180921679*s^3)/144115188075855872 + (1341080270390653*s^4)/144115188075855872 - (5867715276522173*s^5)/9223372036854775808 - 1819839641949093/72057594037927936)^2)^(3/2))
m =
0.85586995465205551704383696315741
3.3845549086104546062118333757515
6.0644608074070649395928866885874
0.86774544065327612268034185895668 - 0.28338346625708125900409850345349*i
0.86774544065327612268034185895668 + 0.28338346625708125900409850345349*i
5.9090579197766355323813767511608 - 3.2214680329909146257449746295832*i
5.9090579197766355323813767511608 + 3.2214680329909146257449746295832*i
6.2175092150938920522167229879549 + 0.81988112353725460786890788489671*i
6.2175092150938920522167229879549 - 0.81988112353725460786890788489671*i
[ empty sym ]
It is "[ empty sym ]"
Do you think it could be the version I used is MatLab R21011b?

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