For (a), remember the formula
For (c), note that Then,
These formula are correct in whatever quadrant x is.also can anyone tell me how to derive the following formulas from the four quadrants mtd....(as i dun want to memorise the formulas)
Draw a circle. Take a point x of the circle in quadrant 1 (it's in general easier). Remember that its abscissa is cos(x) and its ordinate is sin(x).1) cos(-x)=cos x
2) tan(-x) = -tan x
After that, find on the circle where -x is, that is to say the point forming an opposite angle comparing to x.
Find where 90-x is, etc... It's the way to remember that (yes, because you'll have to remember these formula, no matter what you want )