ガチの厳密解の算出に成功！！！
悪魔のような数式が出てくるのでご注意下さい。

exp(2π/11)=cos(2π/11)+i*sin(2π/11)=

-1/10
+1/40 ( - 1 + √(5) + i√(10 + 2√(5)))*(-11/4(89+25√(5)+(45√(5-2√(5))-5√(5+2√(5)))i))^(1/5)
+1/40 ( - 1 + √(5) + i√(10 + 2√(5)))*(-11/4(89-25√(5)+(5√(5-2√(5))+45√(5+2√(5)))i))^(1/5)
+1/40 ( - 1 + √(5) - i√(10 + 2√(5)))*(-11/4(89+25√(5)-(45√(5-2√(5))-5√(5+2√(5)))i))^(1/5)
+1/40 ( - 1 + √(5) - i√(10 + 2√(5)))*(-11/4(89-25√(5)-(5√(5-2√(5))+45√(5+2√(5)))i))^(1/5)

+i/10√(55
-5*(-11/4(89+25√(5)+(45√(5-2√(5))-5√(5+2√(5)))i))^(1/5)
-5/4 ( - 1 + √(5) - i√(10 + 2√(5)))*(-11/4(89-25√(5)+(5√(5-2√(5))+45√(5+2√(5)))i))^(1/5)
-5*(-11/4(89+25√(5)-(45√(5-2√(5))-5√(5+2√(5)))i))^(1/5)
-5/4 ( - 1 + √(5) + i√(10 + 2√(5)))*(-11/4(89-25√(5)-(5√(5-2√(5))+45√(5+2√(5)))i))^(1/5)
)


=0.841253532831181168861811648919367717513292498420537898642650117... + 0.540640817455597582107635954318691695431770607898113840035749889... i