[cos(1)-cos(9)]*[cos(2)-cos(8)]*...*[cos(9)-cos(1)]=? please help!

One of the factors in your expression will be
[cos(5)-cos(5)]

This is zero and by the zero product
principle anyting times zero is zero so the answer is
0

[cos(1)-cos(9)][cos(2)-cos(8)][cos(3)-cos(7)][cos(4)-cos(6)][cos(5)-cos(5)][cos(6)-cos(4)][cos(7)-cos(3)][cos(8)-cos(2)][cos(9)-cos(1)]=0