A(x0) in4(x+△x)-4(x) da(x Ax→>0 △x 11(0 12(0 2(x0) a21(x0)a2(x0) 2n(~0 am (ro)a m2( mn0 0 0 0 0 11 0 12 0 1 0 21 0 22 0 2 0 1 0 2 0 0 d ( ) ( ) ( ) ( ) lim d ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) x x x n n m m mn A x A x x A x A x x x a x a x a x a x a x a x a x a x a x  → = +  −  = =            =           