若1991＜x<1999.则（x-1的绝对值减x-1991的绝对值）＊（x-1990的绝对值加x-2000的绝对值）等于多少
(|x-1|-|x-1991|)*(|x-1990|+|x-2000|)
=[x-1-(x-1991)]*[x-1990+2000-x]
=[x-1-x+1991][10]
=1990*10
=19900