jika U7 = 32 U11=48 maka S20= ?

Un = a +(n-1)b
U7 = a + 6b = 32
U11 = a + 10b = 48

lakukan eliminasi untuk mencari nilai b
a + 10b = 48
a + 6b = 32
_________-
4b = 16
b = 4

maka
a + 6b = 32
a + 6.4 = 32
a + 24 = 32
a = 8

Sn= n/2 (2a + (n-1)b)
S20 = 20/2(2.8 + (20-1)4)
S20 = 10(16 + 19.4)
S20 = 10(16+76)
S20 = 10.92
S20 = 920

Cek gambar yaah. Uraikan Un.nya terelebih dahulu kemudian pake cara eliminasi.