求 Fibonacci 数列

可能溢出的程序：

>> a=[1 1]; for i=3:100, a(i)=a(i-1)+a(i-2);end; a(end)
 
ans =
 
   3.5422e+20

采用符号运算避免溢出：

>> a=sym([1 1]); for i=3:100, a(i)=a(i-1)+a(i-2);end;
>> a(end)
 
ans =
 
354224848179261915075

>> a=sym([1 1]); for i=3:1000, a(i)=a(i-1)+a(i-2);end;
>> a(end)
 
ans =
 
43466557686937456435688527675040625802564660517371780402481729089536555417949051890403879840079255169295922593080322634775209689623239873322471161642996440906533187938298969649928516003704476137795166849228875