On a C2 revision sheet I've come across this circle with a set of perpindicular lines drawn on it to divide the circle up into four segments and the letters S A T C written in each segment (S was top left and the rest followed in clockwise order) and it seems to be something to do with trig (therefore s would be sin, c would be cos etc.)...but what does it mean?
It means that, measuring anticlockwise from the x-axis:
1st quadrant has an A in it. From 0 to 90 degrees, All of sin, cos and tan are positive.
( 30 degrees falls into this category, sin 30 is positive, cos 30 is positive, tan 30 is positive).
2nd quadrant has an S in it. From 90 to 180 degrees, only Sin is positive.
(120 degrees falls into this category, sin 120 is positive, cos 120 is negative, tan 120 is negative).
3rd quadrant has a T in it. From 180 to 270 degrees, only Tan is positive.
(240 degrees falls into this category, tan 240 is positive, sin 240 is negative, cos 240 is negative).
4th quadrant has a C in it. From 270 to 360 degrees, only Cos is positive.
(300 degrees falls into this category, cos 300 is positive, sin 300 is negative, tan 300 is negative).
(the 90, 180, 270, 360 have one of the sin, cos, tan zero or undefined (for tan) but that's probably not something to worry about very much in the context of the diagram, it'll be obvious in any calculation!)
i think your talking bout stac! the four segments are sine, tan, all and cos
they are the quadrant that each function is positive in,
sin between 0 and 180
cos between 270 and 90 (not the same as 90 and 270)
tan between 180 and 270 and 0 and 90
this is just a method to help you work out where the other angles are when your calculator only dispalys one