Совсем просто
1) u(n) = (n+1)!/(2n)
u(1) = 2!/2 = 2/2 = 1
u(2) = 3!/4 = 6/4 = 3/2
u(3) = 4!/6 = 24/6 = 4
u(4) = 5!/8 = 120/8 = 15
2) u(n) = (-1)^(n+1)*1/кор.куб.(n)
u(1) = (-1)^2/1 = 1
u(2) = (-1)^3/кор.куб.(2) = -1/кор.куб.(2)
u(3) = (-1)^4/кор.куб.(3) = 1/кор.куб.(3)
u(4) = (-1)^5/кор.куб.(4) = -1/кор.куб.(4)
3) u(n) = 1/(3n-1)(2n+1)
u(1) = 1/(3-1)(2+1) = 1/(2*3) = 1/6
u(2) = 1/(6-1)(4+1) = 1/(5*5) = 1/25
u(3) = 1/(9-1)(6+1) = 1/(8*7) = 1/56
u(4) = 1/(12-1)(8+1) = 1/(11*9) = 1/99