15а - 5 - 3a^2 + a - 12a + 3a^2 = 4a - 5
S_n=(a_1+a_n)/2*n => S_20=(a_1+a_20)/2*20
Решение во вложении-------------------
Pz+qz+p+q=(p+q)(z+1)
pt+qt+p+q=(p+q)(t+1)
pz+qz+p+q
----------------- =
pt+qt+p+q
(p+q)(z+1)
----------------- =
(p+q)(t+1)
(z+1)
--------- = (25+1)/(12+1)=2
(t+1)