# The Equation for solving Matrix is stiffnessmatrix.d == F

The Equation for solving Matrix is stiffnessmatrix.d == F. Where I know the Matrix {d} and [F] but [ stiffnessmatrix] is an 18x18 matrix contain 8 variable i need to solve for. Can anyone provide code to solve this. Mathematica file is here. https://drive.google.com/open?id=1t0aBpL_LVgWcUnxX4sXRRXhyS5WlHuoU

Using the definitions from your notebook.

As the coeffients Subscript[EI, i] appear only linearly, you may try a least squares fit as follows:

u = Subscript[K, global].d /. {Subscript[he, 1] -> 1,
Subscript[he, 2] -> 1, Subscript[he, 3] -> 0.5,
Subscript[he, 4] -> 0.5, Subscript[he, 5] -> 0.5,
Subscript[he, 6] -> 0.5, Subscript[he, 7] -> 1,
Subscript[he, 8] -> 1};
A = D[
u,
{Table[Subscript[EI, i], {i, 1, 8}], 1}
];

coeffs = LeastSquares[A, F]


However, this will throw an error because you defined F as $$17 \times 1$$-matrix while Subscript[K, global].d is a $$18 \times 1$$-matrix. So your F is too short.

• Thanks for the help @Henrik Schumacher. That was my mistake. The [F] should be 18 X 1 - matrix. – Shumon matideas Jan 7 at 8:39
• You're welcome. – Henrik Schumacher Jan 7 at 8:40