Const
  n=10;
var
  a,b:array[1..n] of real;
  s,s2,s3:real;
  i:integer;
begin
  Randomize;
  Write('A: ');
  s2:=0;
  for i:=1 to n do begin
    a[i]:=Random(51)-20;
    Write(a[i],' ');
    s2:=s2+sqr(a[i])
  end;
  Writeln;
  Write('B: ');
  s:=0; s3:=0;
  for i:=1 to n do begin
    b[i]:=Random(51)-20;
    Write(b[i],' ');
    s3:=s3+sqr(b[i]);
    s:=s+a[i]*b[i]
  end;
  Writeln;
  Writeln('cos(A)=',s/sqrt(s2*s3))
end.

Тестовое решение:
A: 23 -16 24 30 -17 9 -15 -7 23 19
B: -18 1 -18 -1 -8 -5 -16 8 14 -9
cos(A)=-0.206955416562861