1) непонятна запись.
2) 7a³-5a³ = 2a³
9a³-5a³ = 4a³ = 2·2a³
11a³-5a³ = 6a³ = 3·a³
an = a1+(n-1)·2a³
3) a-3-(a-1) = a-3-a+1 = -2
a-5-(a-1) = a-5-a+1 = -4 = 2·(-2)
a-7-(a-1) = a-7-a+1 = -6 = 3·(-2)
an = a1+(n-1)·(-2)
