tolong caranya ka.....?

barisan aritmatika
a = 10
b = 30-10 = 20

Un = 10 + (n-1) 20
170 = 10 + 20n -20
170 = 20n -10
180 = 20n
n = 9

Sn = n/2 (2.a + (n-1) b)
S9 = 9/2 (2. 10 + (8) 20)
S9 = 9/2 ( 20 + 160)
S9 = 9/2 (180)
S9 = 9(90)
S9 = 810

phi = 180
810 : 180 = 4,5
S9 = 4,5 phi
S9 = 4 1/2 phi