Expand ##v_2## in an orthogonal base formed by eigenvectors: ##n_1,n_2,n_3,##$$v_2=k_1n_1+k_2n_2+k_3n_3,$$$$Sv_2=\lambda_1\left(k_1n_1+k_2n_2\right)+\lambda_3k_3n_3,$$but the subspace spanned by ##n_1,n_2## and the subspace by ##n_3## do not intersect, so either ##k_1=k_2=0## or ##k_3=0##.
if...