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Eigenvectors

A is a real symmetric nxn matrix with n orthonormal eigenvectors ei\vec{e_i} & corresponding eigenvalues λi\lambda_i. Obtain coefficients aia_i such that

x=iaiei\vec{x} = \sum_{i} a_i \vec{e_i} is a solution to Axμx=fA\vec{x} - \mu \vec{x} = \vec{f} where f\vec{f} is a given vector and μ\mu a scalar NOT equal to an eigenvalue of A.

How on earth do you go about dealing with something as generalised as this. I''ve manged to use what's given to get to

Axμx=i(λiμ)aieiA\vec{x} - \mu \vec{x} = \sum_{i} (\lambda_i - \mu) a_i \vec{e_i} but I have no clue hwo to proceed to find coefficients aia_i.
Write x=a1e1+...+anen\mathbf{x} = a_1 \mathbf{e}_1 + ... + a_n \mathbf{e}_n and dot both sides with ei\mathbf{e}_i to obtain aia_i using orthonormality.

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