single_inter does not find intersection

09282014, 10:33 AM
Post: #1




single_inter does not find intersection
Hi all!
Working with the Geo App on graphically estimating working points, voltages, gains and the like for a tube shortwave HFAmp. (BTW, great App that tends to become one of my all time favorites  thanks a lot guys!) I defined some plotfunc's of the form plotfunc(a*ln(x)+b). Examples: GB plotfunc(1.301*ln(x)+3.406 GC plotfunc(0.9718*ln(x)+2.524 These functions get plotted without any problems. Then I defined a point: GE point(40,0) It also gets plotted fine. Then an element of GC as: GG element(GC,20) Goes well too. Then the line through GE and GG as: GH line(GE,GG) Everything fine yet. Now I try to find the intersection of GH with GB using: GI single_inter(GH,GB) The App cannot find that point. (It does not work using single_inter(GH,GB,GG) or single_inter(GH,GB,GE) either.) But the CAS can readily nsolve this and get the xcoordinate of the intersection. So, what is going on? Any suggestions? Bye, Nick 

09292014, 06:28 AM
Post: #2




RE: single_inter does not find intersection
You´re right. I followed your steps, and I could not get the intersection point.
By the way, how did you find the solution with nsolve command?. I tried to insert the line ecuation and the LN one, like nsolve([y=0,2717*x+10,87 y=1,301*LN(x)+3,406],[x,y]) and the sandclock works without answer. I had to soft reset. I used fsolve instead and it worked ok. Something does not work well in this case. Bug? By the way: where is the fsolve command in the CAS tool boxes? I cannot find it!. 

09292014, 06:34 AM
Post: #3




RE: single_inter does not find intersection
There is indeed a bug. Will be fixed in Xcas (unstable) this week.


09292014, 07:39 AM
Post: #4




RE: single_inter does not find intersection
(09292014 06:28 AM)akmon Wrote: You´re right. I followed your steps, and I could not get the intersection point. Hi! I just nsolved for x, not for x and y. For example: nsolve(1,301*LN(x)+3,406(0,2717*x+10,87) , x ) which immediately delivers 14.6254232747. Substituting this value back to either 1,301*LN(x)+3,406 or 0,2717*x+10,87 will give you the y value of 6.89627249626. Ciao, Nick 

09292014, 07:54 AM
Post: #5




RE: single_inter does not find intersection
(09292014 06:34 AM)parisse Wrote: There is indeed a bug. Will be fixed in Xcas (unstable) this week. OK. BTW, you mean a bug in single_inter, may I assume? (Or in nsolve, as akmon asked?) Anyway, I replaced the LN functions with ATAN functions (with different sets of fitted parameters), since I just needed more or less good approximations of measurement series. With ATAN the intersections could be found, and all related calculations could be traced perfectly. (And so at last I know now, why that %&*$HFAmp turnes to a HF.. attenuator over 50 MHz. :)) Thanks and ciao, Nick 

09292014, 08:04 AM
Post: #6




RE: single_inter does not find intersection
There were 2 problems: one with ln(x) which was replaced by ln(abs(x))+i*arg(x) leading to problems since ln(x) was multiplied by i in the plot parametric equation, another one with mixing exact and numeric data in simplify.
Using atan is safer since it is defined from R to R. 

09292014, 09:56 AM
Post: #7




RE: single_inter does not find intersection
(09292014 08:04 AM)parisse Wrote: There were 2 problems: one with ln(x) which was replaced by ln(abs(x))+i*arg(x) leading to problems since ln(x) was multiplied by i in the plot parametric equation, another one with mixing exact and numeric data in simplify. I understand  thank you very much! Ciao, Nick 

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