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

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 ?

Answer 1

[Ameer Hamza]

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

[Ameer Hamza]

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]

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

solution is a cell array, where each cell contain solution for a combination of a, b, and c.