How to Substitute an array into an equation and solve it ?

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Kalpha.mc 2020-11-15

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https://ww2.mathworks.cn/matlabcentral/answers/648288-how-to-substitute-an-array-into-an-equation-and-solve-it

编辑： Ameer Hamza 2020-11-16

So if i had a = [1 2 3] b = [4 5 6] c = [7 8 9]

and the equation is ax^2+bx+c = 0

How would i substitude the array and solve for x ?

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Answer 1

Ameer Hamza 2020-11-15

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https://ww2.mathworks.cn/matlabcentral/answers/648288-how-to-substitute-an-array-into-an-equation-and-solve-it#answer_544913

编辑：Ameer Hamza 2020-11-15

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For the given value of a, b, and c, the equation have no real solutions. Following shows how to use fsolve() when the equations have solutions

a = [1 2 3];
b = [4 5 6];
c = [-7 -8 -9];
F = @(x) a(:).*x.^2+b(:).*x+c(:);
sol = fsolve(F, rand(3,1))

If you don't have optimization toolbox, then you can use fzero()

a = [1 2 3];
b = [4 5 6];
c = [-7 -8 -9];
sol = zeros(size(a));
for i=1:numel(a)
    F = @(x) a(i).*x.^2+b(i).*x+c(i);
    sol(i) = fzero(F, rand());
end

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Ameer Hamza 2020-11-15

在 MATLAB Online 中打开

No, you are not using subs() correct. Following shows how it needs to be done

syms x a b c
C = a * (x.^2) + (b * x) + c == 0;
x1 = subs(C,{a,b,c},{1,3,5});
solution = double(solve(x1,x));

Ameer Hamza 2020-11-16

编辑：Ameer Hamza 2020-11-16

在 MATLAB Online 中打开

If a, b, and c are arrays

syms x a b c
av = [1 2 3];
bv = [4 5 6];
cv = [7 8 9];
C = a * (x.^2) + (b * x) + c == 0;
solution = cell(size(av));
for i = 1:numel(av)
    x1 = subs(C,{a,b,c},{av(i),bv(i),cv(i)});
    solution{i} = double(solve(x1,x));
end