6l2=(f-S,f-S)=(/,-S)-(S;f-S (f,八)-(S,f)-(S,∫-S 由 于 (S,f-S')=(∑a9k,f-∑q) k=0 ∑a(,f-∑a) k=0 ∑a(,qk)-∑(129k)) k=0 j=0 所以 62=(f-S,f-S)=(/,)-(S;,)2 * * * * * 2  = − − = − − − ( , ) ( , ) ( , ) f S f S f f S S f S * * * = − − − ( , ) ( , ) ( , ) f f S f S f S 由 于 * * * * 0 0 ( , ) ( , ) n n k k j j k j S f S a f a   = = − = −   * * 0 0 ( , ) n n k k j j k j afa   = = = −   * * 0 0 (( , ) ( , ) ) 0 n n k k j k j k j a f a    = = = − =   所以 ( , ) ( , ) ( , ) * * * 2 2  = f − S f − S = f f − S f