jika deret aritmetika U5=15 dan U11=33,maka S20=?

U11 = a + 10b = 33
U5 = a + 4b = 15
-------------------------- -
6b = 18
b = 3

a + 4.3 = 15
a = 3

S20 = 20/2 (2.3 + 19.3)
= 10 (6 + 57)
= 630

Mohon koreksi ulang, semoga membantu.

U5= 15
U11=33
Ditanyakan : S20=...?

kita cari a sama b nya dulu menggunakan rumus Un

•cari b (suku pertama) misal menggunakan persamaan yg kedua
U11 = a+10b =33
U5 = a+4b =15
-------------------------- -
10b -4b = 18
6b = 18
b = 18:6
b = 3


a + 10b = 33
a +10. 3 = 33
a + 30 = 33
a = 33 - 30
a = 3


jawaban :
Sn = ½n (2a+(n-1)b
S20=½.20 (2.3+(n-1) 3
S20= 10 (6 + 19.3)
S20= 10 (6+57)
S20= 10. 63
S20= 630

semoga membantu :D