matrix

题解

代码

const
  mood=1000000007;
var
  n,aa,bb:longint;
  mi,ans:int64;
  l,t:array [0..200001] of longint;
  a,b,f,jc:array [0..200001] of int64;
procedure init;
var
  i:longint;
begin
  readln(n,aa,bb);
  for i:=1 to n do
    read(l[i]);
  for i:=1 to n do
    read(t[i]);
end;

function ksm(x:longint):int64;
var
  sum:int64;
begin
  if x=0 then exit(1);
  if x=1 then exit(mi);
  sum:=ksm(x div 2);
  sum:=(sum*sum) mod mood;
  if x mod 2=1 then sum:=(sum*mi) mod mood;
  exit(sum);
end;

procedure main;
var
  i:longint;
begin
  jc[1]:=1;
  a[0]:=1; a[1]:=aa;
  b[0]:=1; b[1]:=bb;
  f[0]:=1; f[1]:=1;
  for i:=2 to n*2 do
    begin
      jc[i]:=(jc[i-1]*i) mod mood;
      mi:=jc[i];
      f[i]:=ksm(mood-2);
      a[i]:=(a[i-1]*aa) mod mood;
      b[i]:=(b[i-1]*bb) mod mood;
    end;
  for i:=2 to n do
    begin
      ans:=(ans+(((((((((jc[2*n-i-2]*f[n-2]) mod mood)*f[n-i]) mod mood)*a[n-i]) mod mood)*b[n-1]) mod mood)*t[i]) mod mood) mod mood;
      ans:=(ans+(((((((((jc[2*n-i-2]*f[n-2]) mod mood)*f[n-i]) mod mood)*a[n-1]) mod mood)*b[n-i]) mod mood)*l[i]) mod mood) mod mood;
    end;
  write(ans);
end;

begin
  init;
  main;
end.