Skip to main content

Solve multiple equations in one go


Consider the following code:


a1 := 2
b1 := 1
a2 := 1
b2 := 1/2
x1 := 0
y1 := 1
x2 := 0
y2 := 4


cSol = Solve[
a1 (a1^2 b2^2 m^2 + b1^2 (b2^2 - (c + m x1 - y1)^2)) ==
b2 (a1^2 (a2^2 m^2 + b2^2) - a2^2 (c + m x2 - y2)^2), c];
cc = cSol[[All, 1, 2]];
FullSimplify[TableForm[Table[cc[[i]], {i, 1, 2}]]]

which gives the two values $-2$ and $2$ as output.


I then want to do the following equation solving


Solve[(a1^2 m (cc[[1]] - y1) - b1^2 x1)^2 ==

(a1^2 m^2 + b1^2)*(b1^2 x1^2 + a1^2 (cc[[1]] - y1)^2 - a1^2 b2^2), m]
Solve[(a1^2 m (cc[[2]] - y1) - b1^2 x1)^2 ==
(a1^2 m^2 + b1^2)*(b1^2 x1^2 + a1^2 (cc[[2]] - y1)^2 - a1^2 b2^2), m]

for both values of $c$, in one sweep go, and thereby giving me four values for $m$ as output.


How do I do that without having to solve the equation for both c[[1]] and c[[2]] one at the time, but instead do it in one sweep go?


P.S. I used more or less the same technique as when solving for $c$, before I delete my code by mistake, and now I can't figure out how to do it again. :-(



Answer



First of all you may use the solutions for $c$ in their original Rule form as this is more elegant than using parts:


Solve[

(a1^2 m (c - y1) - b1^2 x1)^2 == (a1^2 m^2 + b1^2)*(b1^2 x1^2 +
a1^2 (c - y1)^2 - a1^2 b2^2), m
] /. cSol // Flatten


{m -> -(Sqrt[35]/2), m -> Sqrt[35]/2, m -> -(Sqrt[3]/2), m -> Sqrt[3]/2}



But to do it in one sweep, why not simply do:


eq1 =  a1 (a1^2 b2^2 m^2 + b1^2 (b2^2 - (c + m x1 - y1)^2)) == 
b2 (a1^2 (a2^2 m^2 + b2^2) - a2^2 (c + m x2 - y2)^2);

eq2 = (a1^2 m (c - y1) - b1^2 x1)^2 == (a1^2 m^2 + b1^2)*(b1^2 x1^2 +
a1^2 (c - y1)^2 - a1^2 b2^2);

sol = Solve[ eq1 && eq2, {m, c} ]


{{m -> -(Sqrt[35]/2), c -> -2}, {m -> Sqrt[35]/2, c -> -2}, {m -> -(Sqrt[3]/2), c -> 2}, {m -> Sqrt[3]/2, c -> 2}}



To get the $m$ values you simply do:


m /. sol



$\left\{-\frac{\sqrt{35}}{2},\frac{\sqrt{35}}{2},-\frac{\sqrt{3}}{2},\frac{\sqrt{3}}{2}\right\}$



Comments

Popular posts from this blog

plotting - How to draw lines between specified dots on ListPlot?

I would like to create a plot where I have unconnected dots and some connected. So far, I have figured out how to draw the dots. My code is the following: ListPlot[{{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4,13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full] I have thought using ListLinePlot command, but I don't know how to specify to the command to draw only selected lines between the dots. Do have any suggestions/hints on how to do that? Thank you. Answer One possibility would be to use Epilog with Line : ListPlot[ {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4, 13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full, Epilog -> { Line[ ...

dynamic - How can I make a clickable ArrayPlot that returns input?

I would like to create a dynamic ArrayPlot so that the rectangles, when clicked, provide the input. Can I use ArrayPlot for this? Or is there something else I should have to use? Answer ArrayPlot is much more than just a simple array like Grid : it represents a ranged 2D dataset, and its visualization can be finetuned by options like DataReversed and DataRange . These features make it quite complicated to reproduce the same layout and order with Grid . Here I offer AnnotatedArrayPlot which comes in handy when your dataset is more than just a flat 2D array. The dynamic interface allows highlighting individual cells and possibly interacting with them. AnnotatedArrayPlot works the same way as ArrayPlot and accepts the same options plus Enabled , HighlightCoordinates , HighlightStyle and HighlightElementFunction . data = {{Missing["HasSomeMoreData"], GrayLevel[ 1], {RGBColor[0, 1, 1], RGBColor[0, 0, 1], GrayLevel[1]}, RGBColor[0, 1, 0]}, {GrayLevel[0], GrayLevel...

list manipulation - Selecting multiple columns from a matrix?

Sample data: data = { {{2013, 1, 1}, 24.13, 167.67, 231.82}, {{2013, 1, 2}, 32.15, 170.92, 225.99}, {{2013, 1, 3}, 35.43, 172.68, 221.67}, {{2013, 1, 4}, 36.73, 173.05, 218.32}, {{2013, 1, 5}, 58.19, 165.96, 197.05}, {{2013, 1, 6}, 69.99, 163.50, 187.52}, {{2013, 1, 7}, 71.37, 154.21, 175.58}, {{2013, 1, 8}, 72.51, 149.66, 163.25}}; I want a DateListPlot with three graphs, so for a matrix formed by columns 1 and 2, one for columns 1 and 3, and 1 for columns 1 and 4. At the moment I'm using this code: data2 = Transpose[{data[[All, 1]], data[[All, 2]]}]; data3 = Transpose[{data[[All, 1]], data[[All, 3]]}]; data4 = Transpose[{data[[All, 1]], data[[All, 4]]}]; DateListPlot[{data2, data3, data4}, Joined -> True, Filling -> {3 -> {1}}] but I have a hunch that this can be done more efficiently. I don't like the Transpose s in particular. Any ideas? edit (for extra credit) What if I need to multiply the second column by 2, which in my solution is simp...