Lagrange Method of Diagonalization and Another Method

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I'm studying symmetric forms, and I'm stuck on understanding the Lagrange Method of Diagonalization. To be specific, when I diagonalize a symmetric matrix I thought that I should find that the elements of the diagonal are the eigenvalues of A (A is the matrix of the form in the canonic basis), but then when I compute the Lagrange method, the elements that are on the diagonal are not eigenvalues of the first matrix (that is, the matrix A). Could anyone try to explain what is so wrong here?