学步园

【题意】

求C(q,p)/C(r,s)的值，保留五位小数

【输入】

多组数据，每组数据一行为q、p、r、s

【输出】

对于每组数据，输出一个数表示C(q,p)/C(r,s)的值

将因数分解后约分再从大到小乘除即可

program poj2613;
var
  p,q,r,s,i,j,k:longint;
  ans:extended;
  count:array [0..10001] of longint;

begin
  while not seekeof do
    begin
      fillchar(count,sizeof(count),0);
      read(p,q,r,s);
      for i:=2 to p do
        inc(count[i]);
      for i:=2 to q do
        dec(count[i]);
      for i:=2 to p-q do
        dec(count[i]);
      for i:=2 to r do
        dec(count[i]);
      for i:=2 to s do
        inc(count[i]);
      for i:=2 to r-s do
        inc(count[i]);
      ans:=1;
      for i:=10000 downto 2 do
        if count[i]>0 then
          begin
            for j:=1 to count[i] do
              ans:=ans*i;
          end
                      else
        if count[i]<0 then
          begin
            for j:=1 to -count[i] do
              ans:=ans/i;
          end;
      writeln(ans:0:5);
    end;
end.