M1 Particle on inclined plane

I'm literally confused by 2 things. How did they get -20cos90-a for part a) Isn't it supposed to be -20cos90? Where did they get this 90-a angle from referring to the diagram that is drawn under part a) and why?

Also why is 20cosa in part b) any why?

Resolving perpendicular to the plane, it's -20sin(a), which is the same as -20cos(90-a)



Here you're resolving parallel to the plane, and the component of the 20N force parallel to the plane is 20cosa.

PS, it would be helpful to have the original questions is you need anything further.

I still don't understand why you've used "20" instead of "2g"" to be in front of cos90-a

I don't understand how 20 plays a part here, is there some sort of Z angle we have to draw with 20?

20N has a component that also acts in the plane you're resolving in.

So why are we using 90-a, isn't this angle only related to gravity? And where 20cosa come from (the angle 'a'?)