Question 880328
Use how the angles are related to reference angles from the first quadrant.  This is the general way of using the Unit Circle.


(b) sin(225),  find how this relates to 90 or 180 or 360 degrees and use positions on unit circle to be sure of signs.  {{{180<225<270}}}; your arrow is pointed into the third quadrant.  {{{225-180=45}}} degree difference.  This points in the complete opposite direction from 45 degrees of the first quadrant.  The sign of the sine is NEGATIVE.  
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{{{sin(225)=-sin(45)=-sqrt(2)/2}}}.


(c)  cos(270),  this is between quadrant 3 and 4.
{{{180<270<360}}} and {{{3*90=270}}}, completely vertical pointing downward.  y=-1 and x=0.
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{{{cos(270)=0}}}.