cos

π

5

+cos

2π

5

+cos

3π

5

+cos

4π

5

=cos

π

5

+cos

2π

5

+cos(π−

2π

5

)+cos(π−

π

5

)
=cos

π

5

+cos

2π

5

-cos

2π

5

-cos

π

5

=0．
故答案为0．