reserve h,r,r1,r2,x0,x1,x2,x3,x4,x5,x,a,b,c,k for Real,
  f,f1,f2 for Function of REAL,REAL;

theorem
  [!cos,x0,x1!] = -2*sin((x0+x1)/2)*sin((x0-x1)/2)/(x0-x1)
proof
  [!cos,x0,x1!] = (cos(x0)-cos(x1))/(x0-x1)
    .= (-2*(sin((x0+x1)/2)*sin((x0-x1)/2))) /(x0-x1) by SIN_COS4:18;
  hence thesis by XCMPLX_1:187;
end;
