cosπ/4+cos2π/4+cos3π/4+cos4π/4=?

[ As π/4 is a special angle, cosπ/4 = √2/2 <--- you can find that in your book
cos3π/4 = cos(π - π/4) = - cosπ/4 = (-√2/2) < --- since cos(π - angle) = -cos (angle)]

so, cosπ/4+cos2π/4+cos3π/4+cos4π/4
=√2/2+0+(-√2/2)+(-1)
= 0 + 0 -1
= -1

用計算機la你~~~~~~