cosa+isina = eia
ei2π55 = 1
cos2π5+isin2π55 = 1
-1+z5 = 0
-1-z1+z+z2+z3+z4 = 0
1+z+z2+z3+z4 = 0
1+z+z2+1z+1z2 = 0
-1+z+1z+z+1z2 = 1+z+z2+1z+1z2
solve-1+x+x2x
1-52+z+1z1+52+z+1z = 0
2+2z2+z1-5 = 0
a) solve2+2z2+z1-5z
z = -1-5-2-5-54
z = -1-5+2-5-54
2+2z2+z1+5 = 0
b) solve2+2z2+z1+5z
z = -1+5-2-5+54
z = -1+5+2-5+54
cos2π5+isin2π5=z=-1-5-2-5-54=-1-5-i25+54
cos2π5 = -1-54
cos2a = -1+2cos2a
subst-1+2cos2aaπ5
-1+2cos2π5 = -1-54
solve-1+1-54+2cos2π5cosπ5
cosπ5 = 23+54
0.8090169943749475 = 0.8090169943749475