Dear sigma1,
All you got here is to substitute the values. I think you can do that. For the second part, what you should know is that and
A more detailed discription about trignometric identities could be found at, List of trigonometric identities - Wikipedia, the free encyclopedia
Hope this helps.
hi
Since x falls in the second quadrant , sin is positive and cos is negative
Draw a triangle with sides labelled according to the trigo ratio , and use phythagoras theorem to evaluate the other side .
in this case , hypothenus=13 , opposite side =12, adjacent=5
cos x=adjacent/hypothenus=5/13 but look at red , so cos x=-5/13
for the remaining examples ,just expand and substitute appropriately .