Why are the eigenvalues of upper triangular matrices diagonal elements?

Let n-order upper triangular square matrix A, whose eigenvalue is λ. According to the calculation formula of the eigenvalue of the matrix, there is | a - λ e | = 0, then there is: | a11 - λ A12 A13 a1n|| a22-λ a23 a24 ……… a2n|| a33-λ ………………… a3n|=0|…………………………………… || an-λ...

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