原式=

1

2

[cos120°+cos（-40°）+cos240°+cos（-80°）+cos200°+cos120°]

=

1

2

（-cos60°+cos40°-cos60°+cos80°-cos20°-cos60°）

=

1

2

[-

3

2

+cos（60°-20°）+cos（60°+20°）-cos20°]

=

1

2

[-

3

2

+cos60°cos20°+sin60°sin20°+cos60°cos20°-sin60°sin20°-cos20°]

=

1

2

[-

3

2

+cos20°-cos20°]

=-

3

4