已知a=根号2+1,求a的三次方-a的平方-3a+2008的值,
a=√2+1;
a-1=√2;
(a-1)²=2;
a²-2a-1=0.
所以：a³-a²+2008=a(a²-2a-1)+(a²-2a+1)+2007=2007