reserve r,t for Real;
reserve i for Integer;
reserve k,n for Nat;
reserve p for Polynomial of F_Real;
reserve e for Element of F_Real;
reserve L for non empty ZeroStr;
reserve z,z0,z1,z2 for Element of L;

theorem Th13:
  cos(5*PI/3) = 1/2
  proof
    5*PI/3 = PI+2*PI/3;
    hence cos(5*PI/3) = -cos(2*PI/3) by SIN_COS:79
    .= 1/2 by EUCLID10:23;
  end;
