tolong bantu jawab dengan rumus + jalannya ya teman2... terimakasih :D

Sn = 1/2n (2a+(n-1)b)
S50 = 1/2*50 (2*60+(50-1)10)
= 25 (120+49*10)
= 25 (120+490)
= 25*610
= 15250 kursi

a = 60
b = 10
n = 50
Sn = 1/2 × n(2a + (n-1)b))
= 1/2 × 50(2×60 + (49)(10))
= 25(120 + 490)
= 25 × 610
= 30500