Here's the question you clicked on:
rini94
if the point P(-3/5,y) lies on the unit circle and P is in the third quadrant, what does y equal?
the unit circle is made of cos, sin coords
Since the x value is negative, we are talking left of the y-axis (Q2 or 3). We are told Q# so that means that our y-coord must also be negative. |dw:1331832229558:dw| Consider than creating a triangle by drawing a radius and drawing a line up to the x-axis
sin^2 + cos^2 = 1 ; was my first impression :) which is identical to MrYs instructions
3-4-5 WAS a shortcut , cool lol
Then, from the triangle example (and amistre) you can use pythagorean theorem to find the unknown side (which is also the y-coord) \[y^2=1^2+(\frac{-3}{4})^2\] Solving this gives you 4/5, but you have to remember that since it is in Q3, it has to be negative.