tolong dijawab dong kak no. 7. terimakasih

Deret bertingkat


1....4.....10....19.....31
..3.......6......9.....12
.......3......3......3

a= 1
b = 3
c = 3

un = a + (n-1)b + 1/2 (n-1)(n-2) c
un = 1 + (n-1)3 + 1/2 (n-1)(n-2) 3
un = 1 + 3n - 3 + 3/2 (n² - 3x + 2)
un = 3n -2 + 3/2 n² - 9/2 n + 3
un = 3/2 n²  - 3/2 n + 1
u(n+1) = 3/2 (n+1)² - 3/2 (n+1) + 1
U(n+1) = 3/2 (n² + 2n + 1)  - 3/2 n - 3/2 + 1
u(n+1) = 3/2 n² + 3n + 3/2  - 3/2 n - 3/2 + 1
U(n+1) = 3/2 n² + 3/2n + 1
u(n+1) = 1/2 (3n² + 3n + 2)
...

b.
1....5....`15....31.....53..
....4....10....16.....22
........6......6.......6
a = 1, b =4 , c= 6

un = a + (n-1)b + 1/2 (n-1)(n-2)c

un = 1 + (n-1)4 + 1/2 (n-1)(n-2) 6
un = 1+ 4n - 4 + 3(n² - 3n + 2)
un = 1+4n-4 + 3n²-9n+ 6
un = 3n² - 5n  + 3

u(n+1)  = 3(n+1)² - 5(n+1) + 3
U(n+1) = 3(n²+ 2n+1) - 5n - 5 + 3
u(n+1) = 3n² + 6n + 3 - 5n - 5 + 3
U(n+1) = 3n² + n  + 1