元|·mx l Ax ll 引孔|-‖Ala ≠0 3)‖A+B‖lmax I (A+ B)x a ≤mv l4xl+‖Bcl ≠0 ≤nv I Ax I Bx ll ≠0 x≠0 叫All+‖B0 a x a || Ax || | | max || x ||   =  A a =|  |  || || A B a 3) || + || 0 a x a || ( A B )x || max  || x || + = 0 a a x a || Ax || || Bx || max  || x || +  0 0 a a x x a a || Ax || || Bx || max max   || x || || x ||  + A a B a =|| || + || ||