mathematica 7 中,怎样计算自定义函数的定积分?

题目

mathematica 7 中,怎样计算自定义函数的定积分?
f[x_] := x /; (x >= 0) && (x < 1/2)
f[x_] := 1 - x /; (x >= 1/2) && (x

答案

用Which命令啊,
a = Which[0 <= x < 1/2, x, 1/2 <= x <= 1, 1 - x]
PiecewiseExpand[a]
Integrate[a, {x, 0, 1}]