№1305 6; 10; 14;... d = 14 - 10 = 6
Найти а91
а91 = а1 +90d = 6 + 90 · 4 = 366.
№ 1395 -375; - 75; -15; ... q = - 15 : ( -75) = 1/5
S15 = b1(q^15 - 1)/(q - 1) = -375·((1/5)^15 - 1)/ (1/5 - 1) = -375 ·5·( (1 - 5^15)/4·5^15
-3·5^4(1 - 5^125)/4·5^15= -3(1 - 5^15)/4·5^11/
(√7 - 2)² = 7 - 4√7 +4 = 11 - 4√7
(√7 +(-2))² = 7 -4√7 +4 = 11 - 4√7
(-√5 + (-3))² = 5 + 6√5 + 9 = 14 +6√5