diff函数

diff函数为求导函数，调用格式为diff(f,x,n)，其中f为函数，x为求导变量（f中的其他字母视为常量），n为求导次数，缺省情况下为1.

>> syms b n;>> m=n*n-5*n+b*n*n*n;>> diff(m,n,1)

MATLAB中解释为：

>> help diff DIFF Difference and approximate derivative.    DIFF(X), for a vector X, is [X(2)-X(1)  X(3)-X(2) ... X(n)-X(n-1)].    DIFF(X), for a matrix X, is the matrix of row differences,       [X(2:n,:) - X(1:n-1,:)].    DIFF(X), for an N-D array X, is the difference along the first       non-singleton dimension of X.    DIFF(X,N) is the N-th order difference along the first non-singleton        dimension (denote it by DIM). If N >= size(X,DIM), DIFF takes        successive differences along the next non-singleton dimension.    DIFF(X,N,DIM) is the Nth difference function along dimension DIM.        If N >= size(X,DIM), DIFF returns an empty array.    Examples:       h = .001; x = 0:h:pi;       diff(sin(x.^2))/h is an approximation to 2*cos(x.^2).*x       diff((1:10).^2) is 3:2:19       If X = [3 7 5               0 9 2]       then diff(X,1,1) is [-3 2 -3], diff(X,1,2) is [4 -2                                                      9 -7],       diff(X,2,2) is the 2nd order difference along the dimension 2, and       diff(X,3,2) is the empty matrix.    See also gradient, sum, prod.    Overloaded methods:       char/diff       sym/diff       fints/diff       iddata/diff    Reference page in Help browser       doc diff