reserve x,x1,x2,x3 for Real;

theorem Th5:
  sin(2*x) = 2*sin(x)*cos(x)
proof
  sin(2*x)=sin(x+x) .=sin(x)*cos(x)+cos(x)*sin(x) by SIN_COS:75
    .=2*sin(x)*cos(x);
  hence thesis;
end;
