Tính giá trị biểu thức E = cos 10° + cos 20° + cos30° + ... cos 170° + cos 180°

`cos10^°+cos20^°+cos30^°+cos40^°+cos50^°+cos60^°+cos70^°+cos80^°+cos90^°+cos100^°+cos110^°+cos120^°+cos130^°+cos140^°+cos150^°+cos160^°+cos170^°+cos180^°`

`=(cos10^°+cos170^°)+(cos20^°+cos160^°)+(cos30^°+cos150^°)+(cos40^°+cos140^°)+(cos50^°+cos130^°)+(cos60^°+cos120^°)+(cos70^°+cos110^°)+(cos80^°+cos100^°)+cos90^°+cos180^°`

`=0+0+0+0+0+0+0+0+cos180°`

`=-1`