barisan aritmatika 5, 7, 9, 11 a)suku ke 9 b)jumlah suku ke 18 tolong y

A = 5
B = 7-5 = 2

A.
U9 :
= a + 8b
= 5 + 8x2
= 5 + 16
= 21

B.
S18 :
= n/2 [2n + (n-1) x b]
= 18/2 [2 x 5 + (18-1) x 2]
= 9 [10 + 9 x 2]
= 9 [10 + 18]
= 9 x 28
= 252