a1=10, d=3. Sn=2a1 + d(n-1)/2 * n = 20 + 3( 30 - 1) / 2 * 30 = 1605
S(k-1) = 20 + 3* 14/2 * 15 = 465
S(k-n)=Sn - Sk-1
Sk-n= 1605 - 465=1140
Х1^2+х2^2= х1^2+2х1х2+х2^2-2х1х2=(х1+х2)^2-
-2х1х2=s^2-2m
(x-11)² < √5 (x-11)
(x-11)² - √5(x-11) <0
(x-11)(x-11-√5) <0
x=11 x=11+√5
+ - +
---------- 11 ------------ 11+√5 --------------
\\\\\\\\\\\\\\\\\\\\\\\\
x∈(11; 11+√5)