Question

数列:1,3,6,10,15,21...编程求第n个数是多少?

Answer 1

第n个数是：n*(n+1)/2

Answer 2

var
    i,n,s:integer;
begin
    readln(n);
    s:=1;
    for i:=2 to n do s:=s+i;
    writeln(s);
end.