a2+a4+a8+a10 =84

(a1+d)+(a1+3d)+(a1+7d)+(a1+9d) = 84

4a1+20d = 84  | :4

 a1 + 5d = 21 , но а1 + 5d = a6 =>

   a6 = 21

S(11) = (2a1 + 10d) /2 * 11 = 2(a1+5d) / 2 * 11 = (a1+5d) * 11  = a6 *11

S(11) = 21*11 = 231

Verified answer

A₄=А₁+3d; A₂=A₁+d; A₈=A₁+7d; A₁₀=A₁+9d

84=A₄+A₂+A₈+A₁₀; 84=4A₁+20d; A₁+5d=21; A₁+5d=A₆=21.

S₁₁=(A₁+A₁₁)*11/2=5,5*(A₁+A₁₁)=5,5*(A₆-5d+A₆+5d)=11A₆=21*11=231.

Ответ: S₁₁=231