Cross-sectional area for stress calculations

[PNGeng]

I am having a lot of difficulty calculating the cross-sectional areas necessary for normal/shear stress problems. If the cross-section is a square or circle, I'm fine, but when the problem makes a diagonal section through the object I get really confused on which angle/trig function to use to calculate the cross-sectional area.

Below are a few FBDs of problems I'm working on just to give a better idea of what I'm talking about.

http://i53.tinypic.com/69fjvc.png
http://i51.tinypic.com/2lji5ol.png
http://i56.tinypic.com/2gv4fet.png
http://i52.tinypic.com/2rrw094.png
http://i56.tinypic.com/2e6a1qt.png

Can someone please walk me through how to calculate cross-sectional area for these kind of problems (what angle/trig functions to use, etc)

 

[NinjaGodel]

Treat the cross sectional area needed as the hypotenuse of a triangle. You know the angle theta. You know the leg A, which is the horizontal area of the figure.

 

[PNGeng]

Thanks for the response. I think I am starting to get the hang of it.