Question 588902
<pre>
sin(x) has a period of 2<font face="symbol">p</font>, so we may add 2<font face="symbol">p</font> to or subtract 2<font face="symbol">p</font> 
from any angle as many times as we please and the sine of the 
resulting angle will be the same.

Therefore 

sin({{{expr(9/4)pi}}}) = sin({{{expr(9/4)pi}}}-{{{2pi}}}) = sin({{{expr(9/4)pi}}}-{{{expr(8/4)pi}}}) = sin({{{expr(1/4)pi}}}) = sin({{{pi/4}}}) = {{{sqrt(2)/2}}} 

Edwin</pre>