tolong caranya yg gambar diatas ya

n = 15, a = 16, b = 4

Un = a + (n - 1)b
U15 = 16 + (15 - 1)4
        = 16 + 56
U15 = 72

Sn = (n/2) (a + Un)
S15 = (15/2) x (16 + 72)
       = (15/2) x 88
       = 15 x 44
       = 660 kursi

16+20+24+28+32+36+40+44+48+52+56+60+64+68+72=660 kursi

mohon maaf bila salah