Aerodynamic Moment at the Leading Edge
 

I have two questions in hand, both related to Chapter 1 of Fundamental of Aerodynamics (Anderson).

In the process of deriving the moment around the leading edge, i.e. M'(LE), minus signs seems to be added in the x-dimension in dM'u and dM'l. I think it is related to sign convention depicted in Figure 1.20. But I cannot understand exactly how +/- sign is decided talking about the normal and axial forces acting on upper and lower body.

Moreover, in Figure 1.22, I cannot find an explanation why a triangle is drawn in that way (similar to the "4th quadrant in trigonometric graph"), to make sin(theta) a negative number?

Thanks for your answers! :)