Question 1162185
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{{{sin(A)cos(B)-cos(A)sin(B) = sin(A-B)}}}<br>
Plug in the numbers:<br>
{{{sin(4pi/7)cos(5pi/21)-cos(4pi/7)sin(5pi/21) = sin(4pi/7-5pi/21)}}}<br>
You can finish....<br>